Is This Prime? is probably the most infuriating, addictive, revolting and unbearably simple games that has ever disgraced my computer screen. I love it, hate it, am glad of its existence and wish it had never been written.
It's pretty tough to think of a simpler premise: you're given an odd number and have to click yes (or press 'y') if it's prime, or no ('n') if not. If you get one wrong, or run out of time (you start with one minute), the game is over.
Evil, evil, evil, as only something that's the brainchild of someone born Christian Perfect can be.
I know perfectly well that 21 isn't prime. It's ingrained into my very being. Yet... the 'y' key gets mysteriously pressed much more frequently1 than I'd like. I don't mind dying on 221 (composite) or 223 (prime), but I routinely get 5 wrong. And I press the space bar to restart the game, cursing the house of Lawson-Perfect, until I make another preposterous blunder, then (while infuriated) claim that 15 is prime, swear again and then carry on.
Is that what Christian had in mind? To distract the minds of mathematicians the world over? (It's been recommended by Matt Parker and Alex Bellos, and featured quite highly on Hacker News and Reddit).
Is This Prime? isn't so much a test of your prime knowledge as it is a test of your decision-making under pressure and your reflexes; as a former Tetris wizard, I pride myself in both -- and yet, even when I get a terrific score (68 is my current best), I feel hard done by.
It's a game that's never beaten. It's a game that's not at all sophisticated, in terms of game play or design. It makes me feel bad about myself.
But I can't stop playing it, and it's only fair that I should let it ruin your life, too.