Let me add a couple more teasers to liven things up and get the brain working:

-- What mathematical symbol can be put between 5 and 9, to get a number bigger than 5 and smaller than 9?

-- Two boys wish to cross a river. The only way to get to the other side is by boat, but that boat can only take one boy at a time. The boat cannot return on its own, there are no ropes or similar tricks, yet both boys manage to cross using the boat.