An Absent-Minded Professor

A notoriously absent-minded professor was one day observed walking along the street with one foot continually in the gutter, the other on the pavement.

A pupil meeting him said:

"Good evening, professor. How are you?"

"Well," answered the professor, "I thought I was all right when I left home, but now I don't know what's the matter with me. I've been limping for the last half-hour."