Alistair and his sisters Rachel and Abigail, all from Histon and Impington Infants School, sent a joint solution:
Rachel and Abigail discussed the shapes, and said that shape B would fall down - because balancing on the pointy bits would be hard.
Shape F would roll down.
Shape I has only got one "leg" at one edge, and there is no weight on the other side to balance it. (Their mum believes they were thinking of a T shape without the right arm.)
Shape I has a fulcrum at the top right, and a weight on the top left so the cuboid will turn downwards and fall over.
Shape D has sort of wheels so if it is made on a flat surface it should be OK, but if it is tilted, the cuboid will roll off.
Rachel and Abigail suggested that if the shapes were given a little "shove", E might fall over. (Mum's translation: unstable)
Alistair, Rachel and Abigail don't mention shapes A, C, G or H. I wonder if this is because they think these are stable?Adriana's son from Australia sent in this lovely picture which shows solutions and ways of fixing the unstable ones.