This week I made a game which has turned out to be like crack to maths fans - http://isthisprime.com/game/. You're shown a number, and you have to decide if it's prime or not. The aim is to correctly sort as many numbers as possible within a minute; the game ends as soon as you get one wrong. So, I thought that would appeal on its own merits to math-funsters, but I have a question which occurred to me while talking about it with Colin Wright: since primes become rarer the higher you go, at what point is just clicking "No" as good a strategy as doing any thinking? I suppose I could rephrase and slightly change the question to: if the maximum number you can be shown increases by a factor of X each time you make a correct decision, what's the expected score of a strategy that just clicks "no" every time? And finally, I'm collecting data on each attempt made at the game. I expect to be able to come up with some interesting statistics about people's intuitions on primality. So far, 51 seems to be by the far most "primey" composite, before factoring in the likelihoods of particular numbers being shown.