Prime numbers are numbers that are only divisible by itself and 1.

1 is a prime because it can only be divided by itself and 1.

2 is a prime because it can be divided by 1 and 2.

3. is a prime because it can be divided by 1 and 3, but not 2.

4 is not a prime because it can be divided by 1, 2, and 4.