Walter Hunt sat in his workshop in 1849, twisting a piece of brass wire. He owed a friend fifteen dollars. He needed to invent something he could sell. In about three hours, Hunt bent the wire into a shape that would change everyday life. His design used a single piece of wire. One end was coiled into a spring. The other end had a clasp that covered the sharp point. The spring created tension that held the pin closed. This simple mechanism made the pin safe to wear against skin. On April 10, 1849, Hunt received a patent for his invention. But he needed money right away. He sold the patent to the W.R. Grace Company for just four hundred dollars. The company made millions from the design. The safety pin works because of basic physics. When you press the pin open, the coiled spring stores energy. When you release it, the spring snaps the clasp shut over the point. This is the same principle behind many tools. A mousetrap, a clothespin, and a hair clip all use spring tension.