This demonstration seems to work best when the surfaces
are only a few ball diameters apart. If the cinder block is
resting on the floor, stand several feet from the wall and the
block. It should be possible to throw the ball down into the
channel and have it return. For this simple demonstration,
however, there are no exact requirements for the width of the
channel. The technique for throwing the ball into the channel
is also a matter of practice