We have to get over an imaginary river with the help of footbridges symbolized by the rods.
Footbridges are at our service in the number we need, but all of them are shorter than the width of the river and we have no binder (glue or nail, etc.), at all.
The problem can be solved by a special way of arrangement invented by Leonardo da Vinci. If we put the rods together in the way you see it on the picture, we have already built Leonardo's bridge, and we can get over the river. The bridge is stable thanks to forces and their opposite forces affecting between the rods.
Leonardo da Vinci (1452−519) was not only an artist but an axcellent engineer and inventor, too. In 1480 he began thinking of a transportable bridge, which can be built without any aids. This is when his solution on the picture above came into being.
