10 Downing Street in Westminster

You're standing by Downing Street, home of the British prime minister. Through the gates you may spot the famous black door of Number 10, often called the most photographed door in the world. Look closely. A white number ten, a black iron knocker, and below it a brass letterbox engraved First Lord of the Treasury. And it's no longer wooden. After an IRA mortar attack in 1991, it was replaced with a blast-proof steel one.