Where did that ball go?

When our new pathway/patio was installed, the old drainpipe leading to a dry well had to be replaced. Rick, our landscaper, suggested I save the old one for the children. It has become the source of great curiosity as balls are dropped into it and fail to reappear at the other end. Though we've put balls through shorter cardboard tubes before this flexible one has them flummoxed!

Gravity continues to be a mystery -