2-3-5-7 is a fantastic game with an easy to understand set of mechanics: swipe right on prime numbers, swipe left on all other numbers. I was shocked to rediscover that 1 is *not* actually a prime number, but once I got past that hurdle the game became very interesting. The difficulty settings are well-tuned, and I was surprised to learn both how easily I developed a sort of innate sense of when a three-digit number was prime and how easily that sense could be led astray. I would highly recommend this for any child learning what prime numbers are, and for any adult who *thinks* they remember what prime numbers are.