A blue laser beam turns on a protein that helps this human cancer cell move. Responding to the stimulus, the protein, called Rac1, first creates ruffles at the edge of the cell. Then it stretches the cell forward, following the light like a horse trotting after a carrot on a stick. This new light-based approach can turn Rac1 (and potentially many other proteins) on and off at exact times and places in living cells. By manipulating a protein that controls movement, the technique also offers a new tool to study embryonic development, nerve regeneration and cancer.