concept

Young's modulus

A measure of a material's stiffness: how much stress it takes to deform it elastically by a given amount, written E and given in gigapascals. It answers "how far does it stretch when I pull," not "when does it break" — stiffness is about shape while strength is about failure, and the two are independent. Steel sits near 200 GPa, bone at 18-22, wood along the grain at 10-13, tendon at only 1.2-1.8. A rubber band has very low stiffness, which does not make it weak. For an organism the number that actually decides things is specific stiffness, E/ρ, because every body has to carry its own materials.