This puzzle may ring a bell with some of you. If so, please refrain from jumping in too soon:
Someone shows me 3 closed boxes and tells me that 2 of them are empty and one contains loadsa money.
He tells me I can pick one box and win its contents, if any, so I pick a box.
Then he, who knows which box the money is in, opens one that I haven't picked, shows me it's empty, and asks me if I'd like to switch my choice to the other unopened box.
Just to be clear, he has no hidden agenda in giving me this option, it's not his money, he's not my friend, he doesn't care whether I win it or not.
Should I switch?