CS Unplugged activity

Count the Dots: lesson plan

A teacher-led version of Count the Dots, following CS Unplugged’s original classroom activity: a demonstration at the front of the room, then every student cuts out and works with their own set of five dot cards. About 50 minutes. Leaving it for a sub? Use the box below instead.

For a sub: the 2-minute version

Skip everything below this box. No cards, scissors, or slides needed.

Optional, if you can project: type https://learn.porttack.com/go/dots into the browser’s address bar (not a search box), press Enter, then use the right arrow key to step through. Stop at the slide titled On Your Sheet, then hand out the sheets. Unsure about a slide? Skip the slides: the sheet explains everything.

Hand out the worksheet, one per student. Names on top.
Say: “Today you’re learning how computers count using only 0 and 1. Read the sheet and work on your own. Stuck? Cover cards with a coin or your finger and count the dots you can still see.”
If the teacher left more puzzles, hand the next one to each student who finishes: a message-in-lights puzzle (a joke whose punchline is hidden in binary).
Restless room? Let neighbors work in pairs. Talking about the puzzle is fine.
Collect every sheet, finished or not.

Before the sub day (teacher, or whoever preps the sub folder): print one copy of the worksheet per student, plus keep-going sheets from the New set page: set Copies to your class size and Sets per student to 2 (front and back). That covers a full period even for your fastest students.

Students will be able to

Explain why computers store everything with just two symbols, 0 and 1.
Spot the pattern in the cards: each card has twice as many dots as the card to its right.
Convert a number from 0 to 31 into five-digit binary, and back again.

Materials

Demonstration cards: pages 2 to 6 of the dot cards, printed once. A4 cards with smiley face sticker dots work well too.
One set of five cards per student: page 1 of the dot cards, copied onto card stock. Scissors for every pair.
The worksheet, one per student, plus the answer key (click Show answer key at the bottom of the sheet, or add ?key=1 to its address).
The slides (optional).
Keep-going sheets: a class set from the New set page, two sets per student (front and back), so nobody runs out of work.

Before class

Print the demonstration cards and the student card pages. Cutting out cards takes about 5 minutes of class time; if time is tight, cut them ahead of time and put each set in an envelope.
Print the worksheets.
Pick five students to hold the demonstration cards.

1. Hook (5 minutes)

Ask: “How many symbols do we use to write any number?” (Ten: 0 to 9.) “What if you only had two?”

Show the first two slides. Everything you see or hear on a computer (words, pictures, numbers, movies, and even sound) is stored using just two symbols, 0 and 1. It’s much easier to build a machine that tells apart two things than ten: a wire has electricity or it doesn’t.

2. Demonstration (10 minutes)

Before giving out the worksheet, it can be helpful to demonstrate the principles to the whole group.

You will need a set of five cards with dots on one side and nothing on the other. Choose five students to hold the demonstration cards at the front of the class. The cards should be in this order: 16, 8, 4, 2, 1.

Discussion. As you give out the cards (from right to left), see if the students can guess how many dots are on the next card. What do you notice about the number of dots on the cards? (Each card has twice as many as the card to its right.) How many dots would the next card have if we carried on to the left? (32) The next…? (64)

We can use these cards to make numbers by turning some of them face down and adding up the dots that are showing. Ask the students to show 6 dots (4-dot and 2-dot cards), then 15 (8-, 4-, 2- and 1-dot cards), then 21 (16, 4 and 1)… The only rule is that a card has to be completely visible, or completely hidden.

What is the smallest number of dots possible? (They may answer one, but it’s zero.)

Now try counting from zero onwards. The rest of the class needs to look closely at how the cards change to see if they can see a pattern in how the cards flip (each card flips half as often as the one to its right). You may like to try this with more than one group.

When a binary number card is not showing, it is represented by a zero. When it is showing, it is represented by a one. This is the binary number system.

Ask the students to make 01001. What number is this in decimal? (9) What would 17 be in binary? (10001) Try a few more until they understand the concept.

3. Cut out your own cards (5 minutes)

Hand out the card pages and scissors. Students cut along the dashed lines and lay their five cards out in a row: 16, 8, 4, 2, 1, from left to right.

4. Partner practice (10 minutes)

In pairs, one partner calls out a number and the other shows it with the cards, then writes it in binary. Swap roles. Some to try:

3, 10, 12, 25, 31
What’s the biggest number five cards can show? (31, all face up: 11111.) How could you count higher? (Add a 32 card.)
10101 and 11111 as decimal numbers (21 and 31).
What day of the month were you born? Write it in binary. Find out what your partner’s birthday is in binary.

Walk the room. A student who adds the cards correctly but writes the bits in the wrong order (11000 for 3) is the most common mix-up: the 1 card is always on the right.

5. Worksheet (15 minutes)

Hand out the worksheet. Students can keep their cards on the desk while they work. Show the “On your sheet” slide so everyone knows where to start.

When a student finishes, hand them the next sheet: a message in lights from a class set, so neighbors get different puzzles.

6. Wrap-up (5 minutes)

Exit ticket, on a scrap of paper:

Write 13 in binary. (01101)
What number is 10010? (18)
In one sentence: why do computers use only 0 and 1?

Going further

Older or faster students: what would a sixth card hold? (32.) How many numbers can six cards show? (64, from 0 to 63.) This is the same idea as a fixed number of bits limiting the biggest number a computer can store (AP CSP 2.1).
Next lesson: Secret Messages in ASCII uses the same binary numbers to send letters.
A different way in: Teaching Binary With Coins does the same activity with coins that are heads or tails.