Second. Best. Simile. Ever.

I think prime numbers are like life. They are very logical but you could never work out the rules, even if you spent all your time thinking about them.

It's from a book called, The Curious Incident of the Dog in the Night-Time by Mark Haddon, which I highly recommend. The book is written from the perspective of an autistic teenager, and while this simile may not be generally applicable, it beautifully crystallizes the narrator's predicament.