The Ulam Spiral is made by going around in a square spiral, with a number, j, going up as it goes around. If j is prime, it puts a dot. What is interesting about this is that there are diagonal lines going across it, with breaks in some of them. This is unusual, because if you did this with plotting randomly, it wouldn't create these same lines all the time.
Credit to Numberphile for teaching me this and Wikipedia for the prime number test. http://en.wikipedia.org/wiki/Primality_test https://www.youtube.com/user/numberphile
