Maths

1) Basically prime numbers don't divide by anything.

And that's the best way to think of them. So Prime numbers are all the numbers that don't come up in your times tables:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37

As you can see there an awkward looking bunch (that's because they don't divide by anthing!) For Example:

The only numbers that multiply to give 5 are 1x5

The only numbers that multiply to give 23 are 1x23

In fact the only way to get any prime numbers is 1x ITSELF

All prime numbers after five all end in 1, 3, 7, 9

You find prime numbers by dividing each one by 3 or 7 and if it can't divide it's prime!