How it works
A variable can count. Each time, it takes its old value and adds one.
A counter starts at zero. The new value is the old value plus one.
A rule that puts 1 in the box every time is a bug. It forgets what was in the box, so it always gives one.
Between players, put zero back in the box, so everyone starts fresh.
Worked examples
The bug
- Tavi's rule says: put 1 in the box called stars.
- The first star lands, and the screen shows 1.
- A second star lands, but the screen still shows 1. The rule forgets what was in the box.
The fix
- Take the old value out of the box, add one, and put the new value back.
- The box holds 2, so 2 + 1 = 3.
- The box now holds 3.
Counting up
- The box starts at 0.
- The first star: 0 + 1 = 1. The second star: 1 + 1 = 2. The third star: 2 + 1 = 3.
- The screen counts up, one, then two, then three.
A new player
- Between players, Mina puts 0 in the box called stars.
- The next player's first star: 0 + 1 = 1.
- Everyone starts fresh.
A taste of the worksheets
These are some of the questions on the sets. The printable sheets have more and an answer key.
- Circle every rule that makes a counter count up. put stars + 1 into stars / put 1 in stars / put 0 in stars / take out the old value, add one, put it back
- Stretch: one girl catches nine stars. Mina puts 0 in stars, and the next player catches 2 stars. The screen shows ___.
- Which rule does not make a counter count up? 🧮 set stars to stars plus one / ➕ add one to the old value / put one in the box every time
- Which rule is a bug in a counter? 🔁 take out, add one, put back / 🙈 forget the old value / 🧮 set stars to stars plus one
- Which rule is a bug in a counter? put zero in the box every time / 🚦 start at zero, add one each time / 🧮 set stars to stars plus one
Words to know
- variable
- A box with a name that holds a value.
- counter
- A variable that counts up.
- old value
- What is in the box before the star lands.
- new value
- The old value plus one.
Watch out for
- Putting 1 in the box every time never counts up. It forgets the old value.
- Putting 0 in the box every time keeps it at zero.
- Emptying the box each time loses the count.
- A bug is fine. We find it, fix the rule and try again.
Try it
The box holds 4 and the fixed rule adds one. What does it hold now?
5. 4 + 1 = 5.
The rule put 1 in stars runs three times. What does the box hold?
1. It puts 1 in every time, however many stars land.
What number does a counter start at?
Zero. A counter starts at zero and adds one each time.
The box holds 2, and we add three. What does it hold?
5. 2 + 3 = 5.
For grown-ups
Write zero on a paper. Each time you do a jump, cross out the number and write the old number plus one. After ten jumps, say what the variable holds. Then try the bug: write one every time, and see what happens.
Play the Mission: write zero on a paper. Each time your child does a jump, they cross out the number and write the old number plus one. After ten jumps, ask what the variable holds. Then try the bug: write one every time, and see what happens.