During WWII, there was a German POW camp in Arizona (near but not too near Phoenix). One day a German POW got his hands on a map and saw a river wasn't too far and, if they had a boat, they'd be able to get away. Oh, there were no fences around the camp because it was in the middle of the desert with no place to go.
Well, the Germans secretly built a boat and planned their escape. They carried the boat to the water (or where the water should have been). You see, the rivers in the desert don't always have water in them. (The expression "when it rains it pours" must have been said by someone from the Arizona desert.) It was not rainy season so the river was dry. The Germans had to walk back to the POW camp with their tails between their legs.