Riddle: There was a building with 100 floors. A short man lived on the very top floor, the 100th floor. On sunny days, he would ride the elevator up to the 70th floor, then climb the stairs up the rest of the way. On rainy days, he would ride the elevator straight to his apartment, the 100th floor. Why?
Answer: He is short, so he can't reach the 100th floor button. On rainy days, he can use his umbrella to poke the button.
Riddle: You have a barrel of oil, and you need to measure out just one gallon. How do you do this if you only have a three-gallon container and a five-gallon container?
Answer: Fill the 3-gallon container with oil and pour it into the 5-gallon container. Then fill the 3-gallon container again and use it to fill the 5-gallon container the rest of the way. Out of the 3-gallon oil, 2-gallon will be required to fill the 5-gallon container completely. Hence one gallon will be left in the 3-gallon container.