Lecturer joke

Next week I have a surprise...

Here's an old joke (obviously pre-Internet) my late brother, who was a lecturer, was fond of:

A lecturer comes in one day with a cassette player. "I have prerecorded lectures for you and the AV assistant will cue them up so for the rest of the semester I don't need to come to the lecture theatre." He presses play and leaves.

Next week, he's curious about how his students are adapting to the arrangement, so at 15 minutes past the hour he peeks into the theatre. There on the lectern is his player, and on the student benches are their cassette recorders, capturing his lecture.