Sorry for nitpicking but you need a puzzle that is knowledge-agnostic (be it cultural or scientific), otherwise you're guarding your site from both bots and people unfamiliar with the concept of or lacking the pre-existing knowledge necessary to solve the puzzle.
What colour is snow is close but you can't assume that everyone knows what snow is, let alone what colour it is. This includes both people with disabilities and in parts of the world where there is no snow...
What colour is snow is close but you can't assume that everyone knows what snow is, let alone what colour it is. This includes both people with disabilities and in parts of the world where there is no snow...