Primes: Send Each Falling Ball into the Right Bucket
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What is Primes?
A numbered ball falls from the top, one at a time. Two buckets wait below: one for primes, one for everything else. You swap their positions so the falling number lands where it belongs, which means doing the mental arithmetic while your hand is already moving.
How to play
Read the number on the falling ball and decide whether it is prime. Hold the mouse button and release it to swap the two buckets around. Send a prime into the prime bucket and a divisible number into the other one. Every correct sort scores, while a ball in the wrong bucket costs you a life or ends your run. The balls speed up as you go, so the time you get to decide keeps shrinking.
Controls
| Key | Action |
|---|---|
| Left click | Hold and release to swap the buckets |
| Tap | Touch the screen to swap buckets on a phone |
Tips
- No even number except 2 is prime, so anything ending in 0, 2, 4, 6 or 8 goes straight to the other bucket.
- Of the numbers ending in 5, only 5 itself is prime; the rest divide by 5.
- If the digits add up to a multiple of 3 the number does too, which catches traps like 21, 27 and 51.
- When you hesitate, test 2, 3, 5 and 7; for small numbers those four checks settle it.
Frequently asked questions
What is a prime number?
A number above 1 that divides only by 1 and itself, such as 2, 3, 5, 7, 11 and 13. This game drills exactly that.
Do the numbers get huge?
They stay small enough to work out in your head. The difficulty comes from the balls falling faster, not from the size of the numbers.