Prime Numbers

Prime Numbers are divisible only by themselves and one.  An easy way to visualize this is, if you had a prime number of sugar cubes you couldn't arrange them into a rectangle or square no matter how much you tried.  Twelve cubes can be arranged either in a 2x6 or 3x4 pattern, but you can't do anything with 13 except 1x13.  Primes may not seem very interesting or useful but they are the basis of most modern methods of encryption.  Prime numbers allow you to buy a book or transfer money safely online.  The prime numbers between 1 and 50 are:

1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41 and 43.

More on prime numbers can be found at wikipedia.