A redhead, a brunette and a blonde were lunching together, discussing what they thought the worlds greatest invention was.

The redhead says, "I think its the airplane, because it has made the world smaller and brought the international community closer together."

The brunette says, "Well, I think it's the telephone, because you can be connected to anywhere in the world in seconds, and it created the international community."

The blonde says, "I think it is the thermos flask."

The other two are confused, "Why?' they both asked.

"Well," says the blonde, "It keeps hot things hot, and cold things cold."

"So?" asks the other two.

"Well, how does it know?"' says the blonde.