A prime number can be divided, without a remainder, only by itself and by 1. For example, 17 can be divided only by 17 and by 1.
Some facts:
The only even prime number is 2. All other even numbers can be divided by 2.
If the sum of a number's digits is a multiple of 3, that number can be divided by 3.
No prime number greater than 5 ends in a 5. Any number greater than 5 that ends in a 5 can be divided by 5.
Zero and 1 are not considered prime...