A rectangular wooden tube
which can be opened flat. A solid wooden block having holes on two of
its opposite sides is imprisoned in the tube by a wooden stake through
the side of the tube.
The block is always visible
from the holes. On command the block visibly penetrates through the
wooden stake and falls from the tube.
method is very clever!
The tube measures cm 10 (l) x 21,5 (h) x 10 (w) (in. 3,94 x 8,50 x 3,94).