You never know until you look.

I never heard of Schrödinger's cat until this morning when somebody shared this Terry Pratchett quote about a very mean cat named Greebo, and it made me giggle:

Greebo had spent an irritating two minutes in that box. Technically, a cat locked in a box may be alive or it may be dead. You never know until you look. In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.