Answer:
What goes up the hill and down the hill, And spite of all, yet stand still?
The more that there is of this, the less you see. What is it?
How can you place a pencil on the floor so that no one can jump over it?
How is it possible to always find what your looking for in the last place you look?
Twelve pears hanging high, twelve men passing by. Each took a pear and left eleven hanging there. How can this be?
They're actually great swimmers now, but later they will become excellent hoppers.