The challenge is to build a bridge using everyday and recycled objects to bear a 1kg (about 2lb) weight.
I offered a variety of objects: a pad of A3 paper, knives (the flat dining room sort!), baked bean tins, some discarded flooring tiles and a 1kg weight.
The paper was quickly found not to work.
There was a little experimentation with the knives balanced on the baked bean tins.
This structure supported the weight adequately although it wasn't up to the children's aim for their bridge.
The flooring tiles turned out to produce the best bridge and could support rather more than 1 kg.
It not only supported the weight of the younger two but could carry Middle Son.
We then looked at the Ladybird book of Bridges which led to the addition of an arch.
I'm not convinced that a cardboard arch added much to the structure!
Other alterations involved extending the bridge with more tins and tiles and using it for playing with dolls and ponies.
Do look at the link up on the sites above for more bridge ideas. This certainly provided some thought and discussion here.