OK, sorry for the delay, here's the next one:
There are 3 light bulbs in a room and 3 switches outside the room. Each switch corresponds to a bulb. Now you are standing outside the room and you want to know which switch corresponds to which bulb. You can enter the room only once. You cant see the switches and the bulbs simultaneously from the doorway, and all the bulbs are in full working order. How will you find which switch stands for which bulb?