It was discovered that if clay was used, rather than mud, very strong bricks could be made by "firing" (or baking) them in a "kiln" (a special oven). "Fired" bricks were like pottery; they did not wash away easily in rain and would sometimes last for thousands of years. The bricks were joined to each other with a type of cement called "mortar". Some houses built by the Ancient Romans are still standing 2000 years later.