Every whole number is either even or odd. The test is simple: try to pair the things up two-by-two. If they pair up perfectly with none left over, the number is even. If there is always one left over, the number is odd.

An even number also splits cleanly into two equal groups — that is the same idea seen from the other side. Counting the even numbers is just skip counting by 2:

Press play, then replay it: each time it takes a new random number of dots and tries to pair them up. Watch whether the last dot finds a partner, or stands alone.

There is a quick shortcut you do not even have to count for: look at the last digit. A number is even when it ends in 0,\ 2,\ 4,\ 6 or 8, and odd when it ends in 1,\ 3,\ 5,\ 7 or 9. So 34 is even and 57 is odd — no pairing needed.

Khan Academy introduces even and odd numbers here: