With a prepared transparency, reveal additional information by moving the mask, placing a second transparency over the first as an overlay, or moving to a new transparency by sliding the old one out as you replace it with the new one. With a piece of clear acetate covering the transparency, write on the extra acetate to add information informally to the image. If you are using a blank transparency, write or draw your image directly on the blank acetate. To call the students' attention to a specific part of the image, place a pencil or a small pointer directly on the transparency rather than pointing toward the screen. When you want the students to turn their attention back to you, turn the projector off.
Overhead projectors are fairly reliable pieces of equipment.
They usually work unless the light bulb has burned out. If the bulb burns out, newer projectors have a back-up bulb which takes over when you flip a switch. If both bulbs are burned out or if you have only a single bulb and it has gone bad, you can change the bulb. Many schools keep a replacement bulb with every projector, or you may have to send to the office to get one. Be sure that the new bulb is of the same wattage as the old bulb.