Finding a prime number to test your computer
Prime numbers are those numbers divisible only by itself and one. For centuries mathematicians calculated them hoping to make some kind of a breakthrough that might prove them to be useful. A Mersenne prime is a prime number that is also all ones in binary (3 is 11 binary, 7 is 111, etc.) Up until the 19th century, there was little use for either flavor of prime numbers, and calculating these numbers was a purely mathematical exercise until cryptographers discovered how to use them to create nearly unbreakable codes.