Nacre is an ultratough natural nanocomposite found in mollusc shells. It consists of hard aragonite nanocrystals and a soft biopolymer matrix. Apart from maintaining the integrity of nacre's brick-and-mortar nanoarchitecture, the biopolymer plays a critical role in the strengthening and toughening of nacre. By directly probing the biopolymer strands using atomic force microscopy, it is revealed that the biopolymer in nacre has the capability to strengthen itself during deformation. This remarkable deformation-strengthening mechanism contributes significantly to the ultrahigh toughness of nacre, and can be explained by a coiled-spring model. The findings explain the mystery of nacre's toughening mechanisms, provide additional design guidelines for developing biomimetic nanomaterials, and lay a constitutive foundation for modeling the deformation behavior of nacre.