Through single-molecule biomechanical experiments, researchers have revealed in unprecedented detail how an intracellular express delivery service works, and why it is so efficient. With tools including optical tweezers, they manipulated a special type of kinesins, transport proteins that "walk" along intracellular fibers carrying vital substances. They found that of the molecule's two "legs" -- made of two different protein chains -- one puts the brakes on its uninhibited partner when there's no cargo attached.