Living materials respond to stresses, or deformation forces, in profound and surprising ways. Bones become weak if they don’t carry weight. Muscles and soft tissues atrophy, or shrink, if exercise is limited. Whether stem cells differentiate into hard or soft tissue can strongly depend on the stresses they experience during growth. Therefore, to promote or direct the growth of healthy tissue—the challenge of tissue engineering—or to limit the growth of unwanted tissue such as cancerous tumors, scientists must understand the stresses that biological materials experience as they grow and live.