Four little gnomes are on one side of a scary wood. They have to travel through the wood to the other side. They can only do this safely if they have some light. Together they carry only one small candle. Unfortunately this candle only gives enough light for two gnomes. Some of the gnomes can walk faster than others. One of them can at fastest walk to the other side in 1 minute, one in 2 minutes, one in 5 minutes and the slow one in 10 minutes. If two gnomes are on the other side of the wood, one of course has to go back to bring the candle. If the gnomes go to the other side as quickly as possible, how much time does it take them in total?

A particular house has two rooms. If you're in one of the rooms you cannot look into the other. In one of the rooms there are three lamps which are connected to three switches located in the other room. For each lamp there is a separate switch. You are in the room with the switches. You are allowed to go only once in the room with the lamps. How can you find out to which lamp each switch is connected?

Suppose you sit in a rubber boat in a swimming pool. In the boat lies a stone. You throw this stone from the boat into the pool. Will the water level in the pool raise or drop?