Lobsters usually develop one smaller, cutter claw and one larger, crusher claw. To show that exercise determines which claw becomes the crusher, researchers placed young lobsters in tanks and repeatedly prompted them to grab a probe with one claw – in each case always be same, randomly selected claw. In most of the lobsters the grabbing claw became the crusher. But in a second, similar experiment, when lobsters were prompted to use both claws equally for grabbing, most matured with who cutter claws, even though each claw was exercised as much as the grabbing claws had been in the first experiment.