The sequence of numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, ...
Each number equals the sum of the two numbers before it.
So after 1 and 1, the next number is 1+1=2, the next is 1+2=3, the next is 2+3=5 and so on.
Appears in many areas of mathematics and in nature, too.
