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Computer Science for Kids

A computer is a machine that follows instructions. It is very fast and never gets tired, but it cannot think for itself.

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What's inside

💻 1 · What Is a Computer?
A computer is a machine that follows instructions. It is very fast and never gets tired, but it cannot think for itself.
🪜 2 · Algorithms
An algorithm is just a list of steps in the right order. You already use them: brushing your teeth is an algorithm!
🔁 3 · Loops
Why write the same instruction eight times when you can say 'do this four times'? That's a loop.
❓ 4 · If This, Then That
Programs make decisions. A conditional asks a yes-or-no question and picks what to do next.
📦 5 · Variables & Data
A variable is a labelled box that holds a piece of information, and you can change what is inside as the program runs.
🧩 6 · Functions
Big problems are just small problems stacked up. Functions let you name a chunk of work and reuse it forever.
🐱 7 · Build in Scratch
Time to build something real. Scratch is free block coding from MIT: you drag blocks together and watch your game come alive.
🐍 8 · Python
Blocks were the training wheels. Python is a real language used by scientists, game studios and websites, and it reads almost like English.
🌐 9 · The Internet
You tap a link and a page appears in a heartbeat. Here is what actually happens in between.
🛡️ 10 · Safety, Data & AI
Knowing how computers work also means knowing how to stay safe and think critically about what they tell you.

Key ideas

Sample worksheet questions

1. Name an input device.
Answer: Keyboard, mouse, microphone or camera
2. Which is the output when you print a picture?
Answer: The printed paper
3. Is a mouse hardware or software?
Answer: Hardware
4. What are the three steps a computer follows?
Answer: Input, then process, then output
5. What is a computer?
Answer: A machine that follows instructions

These are generated fresh every time. Press New Worksheet in the app for a brand-new set, with a printable answer key.

🔌 20 Unplugged Activities (no computer needed)

Computer science with paper, cards, string and people. No screen, no app and no internet, and nothing you have to go out and buy. Every activity says which computer science idea it teaches.

🥪 The Sandwich Robot  Easy · about 15 minutes
Teaches: Algorithms, and why a computer needs instructions that leave nothing out
You need: Bread · Something to spread · A blunt knife · Paper and a pencil
  1. The child writes down the steps for making a sandwich. Every step goes on its own line.
  2. The grown-up is now a robot. Robots do exactly what the paper says and nothing else.
  3. Read step one out loud and do it EXACTLY. If the paper says 'put the jam on the bread', put the closed jar on top of the loaf.
  4. Carry on down the list, being as literal as you can.
  5. When it goes wrong, hand the paper back. The child rewrites the step to be clearer.
  6. Keep going until a real sandwich appears.
Talk about it: Do not help. The whole lesson is in the moment the child realises the robot cannot guess what they meant.
🤖 Robot on the Grid  Easy · about 20 minutes
Teaches: Sequences and commands: writing a program before you run it
You need: Chalk outside, or masking tape indoors, or 16 sheets of paper · Paper and a pencil · A small toy as the treasure
  1. Lay out a grid four squares by four squares on the floor.
  2. Put the toy on one square. That is the treasure.
  3. One player stands on a corner square and is the robot.
  4. Everyone agrees on four commands: FORWARD, TURN LEFT, TURN RIGHT, PICK UP.
  5. The programmer writes the whole list of commands BEFORE the robot moves at all.
  6. Read the program out one line at a time. The robot does only what is written.
  7. If the robot ends up in the wrong place, fix the program and run it again from the start.
Talk about it: Writing the whole program first is the important bit. Steering the robot square by square is not programming, it is remote control.
📿 Pattern Bracelets  Easy · about 20 minutes
Teaches: Loops: repeating a small pattern many times instead of writing it out
You need: String or wool · Beads, buttons or paper circles in two or three colours · Paper and a pencil
  1. Choose a short pattern of three or four beads, for example red, red, blue.
  2. Write it down once, then write REPEAT 8 TIMES next to it.
  3. Thread the beads following your written pattern.
  4. Count how many beads you threaded in total.
  5. Now write out every single bead one by one on paper, with no repeat.
  6. Compare the two pieces of paper and see which was quicker to write.
Talk about it: That short version with REPEAT is a loop. It is why a program that draws a hundred stars is not a hundred lines long.
🎨 Pixel Pictures  Easy · about 20 minutes
Teaches: How a screen builds a picture out of tiny coloured squares
You need: Squared paper · Coloured pencils or crayons · A ruler
  1. Mark out a grid ten squares across and ten squares down.
  2. Draw a simple picture on the grid, colouring in whole squares only. No part squares and no curves.
  3. Hold it at arm's length and then across the room. Notice how the jagged edges smooth out from far away.
  4. Now draw the same picture on a grid of twenty by twenty squares.
  5. Compare the two. The one with more squares looks better and took longer to fill in.
  6. Write down how many squares each picture used.
Talk about it: Each square is a pixel. More pixels means a sharper picture and a bigger file, which is exactly the trade a phone camera makes.
❓ If This, Then That Cards  Easy · about 15 minutes
Teaches: Conditionals: a computer choosing between two paths
You need: Index cards or paper cut into cards · A pencil
  1. Write six rules, one per card, in this shape: IF it is raining THEN take a coat.
  2. On the back of each card, write ELSE and what happens instead.
  3. Shuffle the cards and put them face down in a pile.
  4. Turn one over and read only the IF part out loud.
  5. Everyone else says what should happen next.
  6. Turn the card over to check.
  7. Now make three cards where the IF part is something silly, and see whether the rule still works.
Talk about it: Ask what happens if the IF part is neither clearly true nor clearly false. Computers hate that, and so will your rules.
🗺️ The Instruction Trail  Easy · about 25 minutes
Teaches: Writing a clear sequence for somebody else to follow
You need: Paper and a pencil · A room or a garden · A small prize to hide
  1. One player hides the prize while everybody else is out of the room.
  2. That player writes a numbered list of instructions to find it, using steps and turns only.
  3. No pointing and no extra hints once the list is written.
  4. Hand the list to somebody else and watch them follow it.
  5. Mark the first instruction that goes wrong.
  6. Rewrite that one instruction so it cannot be misread, then test it on a third person.
Talk about it: Counting steps works far better than 'go a bit further'. Precise words are the whole job.
🗂️ Odd One Out Sorting  Easy · about 15 minutes
Teaches: Data and attributes: describing things so a machine could group them
You need: A pile of mixed objects from around the house, about twenty · Paper and a pencil
  1. Tip all the objects onto the table.
  2. One player secretly picks a rule, for example 'made of metal'.
  3. They sort the objects into two groups following that rule, without saying what it is.
  4. Everybody else guesses the rule.
  5. Write down three different rules that would sort the same pile in three different ways.
  6. Try sorting by a rule that needs a yes or no answer, then by one that needs a number.
Talk about it: A rule a computer can follow must have a clear yes or no answer. 'Is it pretty' will not sort anything.
🔢 Binary Number Cards  Medium · about 25 minutes
Teaches: Binary: counting with only two digits, the way every computer does
You need: 5 index cards or pieces of card · A pencil · Dot stickers, or just draw dots
  1. Make five cards with dots on one side: 1 dot, 2 dots, 4 dots, 8 dots and 16 dots.
  2. Lay them in a row with 16 on the left and 1 on the right. The blank side means off.
  3. To show a number, turn cards face up until the dots add up to it. Always take the biggest card that still fits.
  4. Make the number 5. You should have 4 and 1 face up, so 0 0 1 0 1.
  5. Take turns calling out a number under 32 for the other person to build.
  6. Now write each answer as five 1s and 0s, where 1 means face up.
  7. Count from 0 up to 10 in binary, writing each one down, and look for the pattern in the last column.
Talk about it: Every card is double the one to its right. That doubling is the whole idea, and it is why the eighth card would be 128.
🅰️ Your Name in Binary  Medium · about 25 minutes
Teaches: How letters are stored as numbers
You need: String or wool · Beads or paper squares in two colours · Paper and a pencil
  1. Number the alphabet: A is 1, B is 2, all the way to Z is 26.
  2. Write down the number for each letter of your name.
  3. Turn each number into binary using the card trick from Binary Number Cards, with five digits each time.
  4. Choose one bead colour for 1 and another for 0.
  5. Thread the beads for your first letter, then add a spacer bead, then the next letter.
  6. Carry on until your whole name is on the string.
  7. Swap bracelets with somebody and work out whose name you are holding.
Talk about it: Five digits is enough for 26 letters because 5 doublings reach 32. Four digits would only reach 16, so half the alphabet would not fit.
📏 Bubble Sort Line-Up  Medium · about 20 minutes
Teaches: A sorting algorithm, done with people instead of numbers
You need: 6 to 10 players, or 10 cards with numbers on · Space to stand in a line
  1. Everybody holds a card with a number on it, or just stands in a random order by height.
  2. Start at the left end of the line.
  3. Compare the first two people only. If the left one is bigger, they swap places.
  4. Move one place right and compare the next two. Swap if the left one is bigger.
  5. Carry on to the end of the line. That is one pass.
  6. Go back to the left and do another pass.
  7. Stop when you get all the way along with no swaps at all. The line is sorted.
  8. Count how many passes and how many swaps it took.
Talk about it: Try it again with ten people instead of six and count again. The work grows much faster than the line does, which is why nobody sorts a million things this way.
🔍 Guess My Number: Two Ways  Medium · about 15 minutes
Teaches: Linear search against binary search, and why the strategy matters
You need: Paper and a pencil · Two players
  1. One player thinks of a number between 1 and 100 and writes it down.
  2. Round one: the guesser starts at 1 and counts up, one number at a time. Count the guesses.
  3. Round two: a new number. This time the guesser always says the middle of what is left, starting with 50.
  4. After each guess the other player says HIGHER or LOWER.
  5. Count the guesses again.
  6. Play three rounds of each way and write down the worst case for both.
  7. Work out how many guesses the halving method would need for 1 to 1000.
Talk about it: Halving needs at most 7 guesses for 100 and only 10 for 1000. Counting up needs 100 and 1000. That gap is why search algorithms matter.
💃 Loop Dance  Medium · about 20 minutes
Teaches: Loops and nested loops, written as a program and then performed
You need: Paper and a pencil · Space to move · Music if you like
  1. Invent four simple moves and give each one a short name, such as CLAP, STOMP, SPIN, JUMP.
  2. Write a routine using only those names, for example CLAP CLAP STOMP.
  3. Put REPEAT 4 TIMES around it and perform it.
  4. Now write a second short routine and put REPEAT 3 TIMES around that.
  5. Put both loops inside one more loop that says REPEAT 2 TIMES.
  6. Perform the whole thing and count how many moves you actually did.
  7. Check your count by multiplying instead of counting.
Talk about it: A loop inside a loop is a nested loop. Multiplying to predict the total is exactly how a programmer knows what their code will do before running it.
🐞 Debug the Dance  Medium · about 20 minutes
Teaches: Debugging: finding the one wrong step in a program
You need: Paper and a pencil · Two or more players
  1. Player one writes a routine of about ten moves using the names from Loop Dance.
  2. They perform it once, correctly, while everybody watches.
  3. Player one then copies the routine onto a fresh sheet, changing exactly one step, and hands it over.
  4. Player two performs the new sheet exactly as written.
  5. Everybody else watches for the moment it stops matching what they saw.
  6. Whoever spots the wrong line first says STOP and points at that line.
  7. Fix that line, run it again from the top, and check it matches.
Talk about it: Running it from the top after a fix is the habit worth building. Programmers do it because a fix in one place can break another.
🔐 Secret Message Wheel  Medium · about 25 minutes
Teaches: Ciphers: hiding a message by shifting every letter
You need: Card · Scissors · A paper fastener or a pin and a rubber · A pencil · A ruler
  1. Cut two circles of card, one about 10 cm across and one about 7 cm.
  2. Write the alphabet evenly around the edge of each circle.
  3. Push the fastener through the centre of both so the small circle turns on top of the big one.
  4. Choose a shift, for example 3, and line A on the small wheel up with D on the big one.
  5. Write your message, then find each letter on the small wheel and write down the letter it points to.
  6. Give the coded message to somebody with the shift number.
  7. They set their wheel the same way and read it backwards.
  8. Now try to crack a friend's message without being told the shift.
Talk about it: There are only 25 shifts to try, which is why this code is easy to break. Real encryption uses keys with more possibilities than there are atoms in the room.
📨 Paper Packets  Medium · about 25 minutes
Teaches: Networks: how a message is split into packets and put back together
You need: Paper cut into small slips · Pencils · 4 or more players · A room to move around in
  1. Write a message of about twenty words. Do not show anybody.
  2. Split it up so each slip carries only two or three words.
  3. Number every slip so the order is recorded on the paper itself.
  4. Shuffle the slips and hand them to different players standing around the room.
  5. Each player passes their slip to the next player, and so on, until all the slips reach the person at the far end.
  6. That person puts the message back together using the numbers, not the meaning.
  7. Now play again but secretly lose one slip, and see what the receiver does.
  8. Agree a rule for what the receiver should say when a number is missing.
Talk about it: Numbering the slips is the whole trick. It is why a video still plays properly even though its packets arrive out of order.
🃏 The Card Flip Trick  Hard · about 30 minutes
Teaches: Error detection: how a computer notices that data got corrupted
You need: 36 cards that are a different colour on each side, or 36 squares of paper coloured on one side only
  1. Lay the cards out in a square, five across and five down, with random sides showing.
  2. Now add one extra card to the end of every row, choosing its side so each row has an EVEN number of coloured cards.
  3. Do the same for every column, adding an extra card at the bottom.
  4. Add one more card in the bottom right corner so the last row and the last column are both even too.
  5. You now have a six by six square where every row and every column has an even count.
  6. Turn away while somebody flips exactly one card over.
  7. Turn back and find it: one row and one column will now have an odd count, and the flipped card sits where they cross.
  8. Try it again with two cards flipped and see whether you can still find them.
Talk about it: Those extra cards are parity bits. Notice that one flip is always findable but two flips can hide each other, which is a real limit of this method.
🕸️ Sorting Network on the Floor  Hard · about 30 minutes
Teaches: Doing comparisons in parallel instead of one after another
You need: Chalk outside or masking tape indoors · 6 players · 6 cards with numbers on
  1. Draw six lanes side by side on the ground, each long enough to walk down.
  2. Draw short rungs joining pairs of lanes at different points along their length. Copy a six input sorting network diagram, or use this order: join lanes 1 and 2, 3 and 4, 5 and 6, then 1 and 3, 4 and 6, then 1 and 2, 3 and 5, 4 and 6, then 2 and 3, 4 and 5, then 3 and 4.
  3. Each player takes a card and stands at the start of one lane.
  4. Walk forward together.
  5. When two players meet at a rung, they compare cards. The smaller number carries on down the left lane, the bigger down the right.
  6. Keep walking until everybody reaches the far end.
  7. Read the cards from left to right. They are sorted.
  8. Do it again with different numbers and time it against the Bubble Sort Line-Up.
Talk about it: Several comparisons happen at the same moment here, which is why it finishes so much faster. That is parallel computing, done with feet.
🗺️ Map Colouring Challenge  Hard · about 30 minutes
Teaches: Graph colouring: solving a problem where choices affect each other
You need: Paper · Coloured pencils in at least four colours · A pencil
  1. Draw a squiggly closed shape on the paper, then draw more squiggles across it until you have about twelve regions.
  2. The rule: two regions that share an edge must not be the same colour. Touching at a single corner is allowed.
  3. Colour the whole map using as few colours as you can.
  4. Write down how many colours you needed.
  5. Try again on a new map and see whether you can ever be forced to use five.
  6. Now try to DRAW a map that needs five colours.
Talk about it: You will not manage it. Any flat map can be coloured with four, which mathematicians spent over a hundred years proving, and the proof needed a computer.
📚 The Human Search Engine  Hard · about 35 minutes
Teaches: Indexing: why searching an index beats searching everything
You need: 10 short pages of text, from a magazine or written by hand · Index cards · Pencils · A timer
  1. Number the ten pages 1 to 10 and spread them on the table.
  2. Round one: somebody calls out a word. Everybody searches every page to find which ones contain it. Time it.
  3. Now build an index. Choose about twenty interesting words.
  4. Write each word on its own card, and under it list the page numbers where that word appears.
  5. Sort the cards into alphabetical order.
  6. Round two: call out another word. Find it in the index instead. Time it again.
  7. Compare the two times.
  8. Talk about the cost: how long did building the index take, and how many searches before it pays for itself?
Talk about it: A search engine does exactly this. It reads the whole web slowly and in advance, so that your search can be fast.
🎰 Paper State Machine  Hard · about 35 minutes
Teaches: State machines: a system that remembers where it is and what happens next
You need: A large sheet of paper · Coloured pens · A counter or coin · A dice or a coin to flip
  1. Choose something with a few clear states, for example a traffic light, or a vending machine, or a door that can be locked, shut or open.
  2. Draw a circle for each state and write the state's name inside it.
  3. Draw an arrow from one circle to another for every change that can happen, and write ON the arrow what causes it.
  4. Mark the starting state with a thick arrow coming in from nowhere.
  5. Put the counter on the start state.
  6. Call out events one at a time. Move the counter only if there is an arrow for that event.
  7. If there is no arrow, the machine ignores the event. Say so out loud and leave the counter where it is.
  8. Now find a state you can get into but never get out of, or prove there is not one.
Talk about it: A state you can never leave is called a trap state. Finding one on paper is far cheaper than finding one in a program that is already running.

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