There’s a popular form of art in Japan called etegami, or “picture postcards.” Artists draw or paint a design on the blank side of a postcard, usually accompanied by some written characters and a seal impression, and then mail them to friends. Groups of people often exchange these cards. The seals they use are often very simple and rough, many with only one character. A popular way to make these is to use an exacto knife to cut the seal design into the end of a hard white eraser and use this as a stamp. Since the eraser is easy to cut it also won’t last very long, but makes for a handy informal seal.