See the light

Got an urgent call to go to a classroom as the OHP {over head projector} was not working.

Willy gets there plugs in OHP and switches it on, It works.

Willy: "What seems to be the problem with this?"

Teacher: "Oh when I turned it on there was only a tiny beam of light on the screen."

Willy: "The light adjustment was not set correctly." Shows teacher how to adjust light

Teacher: "Oh"

In University the Teaching students have to give reports and presentations using OHP's, there obviously was no marking component on how to operate them!