Tip of the iceberg

A saying that comes from the observation that 90% of an iceberg is under the water. Basically, the appearence of this phrase in a conversation means that whatever you are talking about is only the beginning of a very big problem.

A man goes to the doctor and says: "Doctor, there's a piece of lettuce sticking out of my bottom." The doctor asks him to drop his trousers and examines him. The man asks, "Is it serious, doctor?" The doctor replies, "I'm sorry to tell you but this is just the tip of the iceberg."

In Richard Wiseman's online collection and rating of jokes study (2001) for the University of Hertfordshire (Laughlab), this was the joke that the British participants found funniest.