A teacher is giving a science class and talking about the Earth and its movements around the Sun. "What's the axis of the Earth?" the teacher asks. And a student answers. "It's an imaginary line which passes from one pole to the other, and on which the earth revolved."
Very good," the teacher replies. "Now, could you hang your clothes on that line?"
"Yes, sir," the student replies. 
Surprised, the teacher responds, "Oh, really? What sort of clothes, if you don't mind me asking?"
And the student replies, "Imaginary clothes."