3n + 1!
Now, if you know maths, you might know this infamous problem.
Just in case you don't here are the rules.
Choose a number. Any number
If it's odd, times it by 3 and add 1
If it's even, divide by 2.
The theory is, any number you chose, it will always get stuck in a 4, 2, 1 loop
It's a really simple code, but a really complex puzzle
Warning: May glitch your computer