The pictures included the face of a man hidden in a mountain, a drawing of a duck that could also be a rabbit, and a series of spirals that appeared to be moving, but actually were not.
The pupils were pointing at the screen and deep in discussion with their neighbours. Some of them talked to me about the illusions when I met them in the playground afterwards.
Presenting bright youngsters with quirky things is a great way of enthusing them. Since its initial outing in assembly, I have used optical illusions, with a data projector, as a backdrop on open day in the physics room. It attracts people best if the slides move from one to the next automatically.
It always elicits some reaction, with older children asking me if I've seen this or that illusion.
The younger siblings who are dragged along to these events have been known to stand and watch the display, mute, ignoring all calls to continue the tour until the sequence returns to the beginning again.
I have also used my optical illusions to illustrate vision and colour perception with my A-level medical physics students. It leads on to discussion of all sorts of other imaging issues, such as perception of motion, depth of field, spatial resolution, dark adaptation or peripheral vision.
I have passed my PowerPoint to art, maths (Escher tessellations seem to attract them) and junior school teachers.
It requires no feeding, it is clean and totally silent (although the children may not be) and it is free. Have fun with it.
Alison Camacho is head of physics at Clifton High School in Bristol.