I mean for this to become a megathread, so if you know any good ones, post 'em! Also, no googling the answers. You are in a house that has two levels: upstairs and downstairs. You are currently downstairs. You have in front of you a panel of three switches. One of them controls the only incandescent light in the house, which is located upstairs, and cannot be seen from downstairs. You are allowed to flip any combination of any switches on, and off, only once, and for an indefinite amount of time. You are allowed to take one trip upstairs. You are allowed to use any sense you possess to check the light while upstairs. Which switch controls the light? Have fun.