Author Robin Sloan offered a simple competition:

give me a prime number of your choosing. I’ll send books to the five people who choose the lowest unique prime numbers. So, if you pick 2 but seven other people pick 2, no book for you. If you pick 3 and no one else picks 3, you get a book.

Which number would you pick?

The primes people chose, including the five winners, and more background information are given in a blog post The Penumbra primes.