The one about the clock comparison...

A young pretty female school teacher had been telling her class about the value of being observant and said, "Now children, look at the clock; what does the clock have that I have too?"

One little girl stood up and said, "It has a face."

Another girl raised her hand and said, "It has hands."

"Splendid," said the teacher, "now what has the clock that I haven't got?"

After a long silence, little Johnny rose and said, "You ain't got no pendulum, Miss."