ELI5: Explain Like I'm 5

fermat's little theorem

Fermat's Little Theorem says that if you take a whole number, like 5, and you multiply it by itself a certain number of times, like 4 times, you will get a new number, like 625. Then, if you divide that number by the original number you started with, like 5, you will always end up with a number that ends in 1. So for example, 625 divided by 5 gives 125 with a remainder of 1.