The world's first postage stamp was issued by Great Britain in 1840. Featuring a bust of Queen Victoria, it has come to be known as the "Penny Black." It also marked the introduction of "uniform penny postage" in a mail system: the rate to carry a letter to ANY destination within the country was now the same, and not based on distance carried.
Quote from The Big Bang Theory 10x05 Sheldon Cooper: When I opened the door, I saw my father having relations with another woman. Penny Hofstadter: Oh, that's awful! Sheldon Cooper: I know. It's also why I never open a door without knocking three times. I mean, the first one's traditional, but two and three are for people to get their pants on.