ABSTRACT
I n order to have a party on the top of a cliff, two people, Pi and P<¿, want
to transport a large number of beer bottles from the ground level to the
top. There are two paths, both with several ups and downs. The bottles
are all placed on a rigid board of sufficient length, and the idea is that P\
takes the first path, P2 the second one. But since the board is slippery,
it should be kept absolutely horizontal all the time, for otherwise the
bottles will slide down. C a n this always be prevented?
