A recipe calls for 4 cups of water. You have only a 3-cup and a 5-cup container. How can you measure out four cups of water?
Fill the 5 cup container full....pour 3 cups into the 3 cup container....throw these 3 cups out.........there will be 2 cups now in the 5 cup container....pour these 2 cups into the 3 cup container.....
Fill the 5 cup container full, again......pour 1 cup into the 3 cup container which is now full.......the 5 cup container will now contain exactly 4 cups.....!!!!
This problem appears in the movie Die Hard 3. Bruce Willis and Samuel L Jackson had 30 seconds to solve it in order to prevent an explosion which would have killed them!
Luckily, the penalty for getting problems wrong here is not so severe!