Mostly likely they will build one using single beams to span the walls.  You can demonstrate how this design fails if the room is bigger than the length of the beams, by asking them to take a small step backwards so they are now more than a beam length apart. If the timber beams are only supported at one end by the walls, how can you arrange them so that together they provide stable, strong roof? The most successful approach with tour groups has been to just stand back and let the group get on with experimenting and solving this puzzle.  If they are struggling after several minutes you could suggest they need to have the beams supporting each other, interweaving them in some way.  The answer will be something like the following construction:     End of first beam rests on one wall (you temporarily support other end)     End of second beam rests on next wall and other end on middle of first beam     End of third beam rests on last wall, other end on middle of second beam, and the centre of third beam supporting end of first beam. To demonstrate that the structure is not only stable but can support significant weights, place several books in the centre of the arrangement (if the beams interlock close to the centre) or book at each of the three points where the beams cross.  Encourage other people in the group to try holding the ends of the beams to