Carbon nanotubes are the stiffest known fiber, with a measured Young’s modulus of up to 1.4 TPa [M.-F. Yu et al., Phys. Rev. Lett. 84, 5552 (2000)]. They have an expected elongation-to-failure of 20-30%, which combined with their stiffness, projects to a tensile strength well above 100 GPa (possibly higher). The tensile strength is difficult to measure, but laboratory studies have given values of up to about 50 GPa [Walters et al., Appl. Phys. Lett. 74, 3803 (1999)]. For comparison, the Young’s modulus of high-strength steel is around 200 GPa, and its tensile strength is 1-2 GPa. Carbon nanotubes can be doubled over, and then snap back with no resulting defect, showing remarkable toughness for something so stiff.