If a number is divisible by 2, the number is even.
A number is divisible by 2 if it ends in 0, 2, 4, 6, or 8.

If a number is not divisible by 2, the number is odd.
A number is not divisible by 2 if it ends in 1, 3, 5, 7, or 9.

If the objects in a group can all be put into pairs without having one left over, then the group has an even number of objects.

If the objects in a group are put into pairs and one is left over,
then the group has an odd number of objects.