Asked by: James Louis, Bristol
Crabs have a wide, flattened shape that makes it easy to burrow into sand or squeeze into narrow crevices, but also restricts the range of motion at the 'shoulder' joint of each leg.
Crabs can actually shuffle slowly forward, but move much faster by flexing the second joint of each leg. These joints are simple hinges, like our knees, and they only bend sideways.
Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.