Riddle: you have 2 bottles. One is a 3L bottle and the other one is a 5L bottle. Using only these two bottles, make a water that contains 4L.
Answer: First, you pour water inside the 5L bottle until its full. Empty the water into the 3L bottle, and you'll have 2L's of water in the 5L bottle. Then empty the water in the 3L bottle completely. Then pour the 2L of water in the 5L bottle to the 3L bottle. Then add water in 5L again, and pour out the water again into the 3L bottle. It will only fill 1L because there is already 2L of water in the 3L bottle. Then you will have 4L of water inside the 5L bottle.