A demonstration of the proportionality of tension and elongation during elastic deformation of a metal wire (Hooke's law). A copper wire, 4.4 m long, 0.4 mm in diameter, is suspended from the ceiling of the lecture hall. For the measurement of its elongation, a mm scale is attached to its lower end and is projected onto a screen, together with a fixed pointer. The wire is pulled in two steps by attaching consecutively two 300 g weights. The total elongation measured is 1 mm for 300 g, and 2 mm for 600 g, and is shown to be fully reversible when the weights are removed, indicating an elastic deformation. Elongation and force are proportional, in compliance with Hooke's law.