Answer to last riddle: Pull the plug on the refrigerator, and then open it. This will even out the lower temperature in the refrigerator with the higher temperature of the room, which will drop a little. The stupidest thing to do is to open the refrigerator without pulling its plug. It will then suck the heat of the room into its cooling compartment and get rid of it again through the cooling ribs on its backside, adding even more heat in the process. Being open, the cooling compartment will never be cold enough to trip the thermostat, and the fridge will be running constantly, in effect acting as an oven.
Wow, you guys saw right through that one! I thought no one would think to pull the plug first...
This next one is a classic!
A surgeon needs to operate on three patients, one after another. But the surgeon has only two individual surgical gloves. How can the surgeon operate on the three patients in turn without risking infection for the patients or for himself?