Fractions

The math teacher was giving a lesson on fractions and wrote an example on the chalkboard.
He explained that the numerator was at the top and the denominator was at the bottom.
Leaning against the board, he asked the class, “Are there any questions?”
When he turned back to face the board, laughter filled the room.
“Mr. Alexander,” one student giggled, “you have chalk dust all over your denominator!”