I used to do this in CS/
Math/
Physics classes when I was bored. You can easily waste a good twenty minutes per class doing this. But I just found out there's actually a name for it. Anyway, here it is...
Take any natural number greater than 0.
If it's odd, multiply it by 3 and then add 1.
If it's even, divide by 2.
Continue this until you reach the number 1.
Example: 13 ==> 40 ==> 20 ==> 10 ==> 5 ==> 16 ==> 8 ==> 4 ==> 2 ==> 1
This is called the Collatz Problem, and has yet to be proven true for all natural numbers (although it has been verified for all numbers up to 5.6 * 10^13.