A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself. The number 7 is prime because the only numbers that divide into it evenly are 1 and 7. The number 12 isn't prime because it can be divided evenly by 1, 2, 3, 4, 6, and 12.

The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, and 23. Notice that 2 is the only even prime number: every other even number can be divided by 2, which disqualifies it from being prime. The number 1 isn't considered prime by mathematicians because of how the definition works.

Prime numbers are the building blocks of all other numbers, like atoms in chemistry. Every whole number can be broken down into prime numbers multiplied together. For example, 12 equals 2 × 2 × 3, all primes. Mathematicians have studied primes for thousands of years, and they're still discovering new things about them. There's no largest prime number: no matter how high you count, there's always another prime waiting to be found. Today, prime numbers help keep your passwords and messages secure online through encryption, turning ancient mathematical curiosity into modern digital protection.