A tennis ball has rolled its way down into a hole. This particular hole is extremely deep and has a sharp bend in the middle, making the ball impossible to retrieve by hand. The ground around the hole is made of hard clay, so digging the ball out isn't an option. What could you use to get the ball out?
Water. Pour water in the hole and it will float to the top for you to get.
How can you get 10 horses in to 9 stables, bearing in mind that each horse must have its own stable?