I think this one will cost you a WHOLE DOLLAR!
In the Health and Beauty area, find the "Trial Sized" bottles of SCOPE mouthwash. They are about fifty cents each. (Buy two, they're small.)
Hold the cap of one up to your eye as though it were a spyglass. Tell the patient you are "Scoping things out." Then hold the bottle up so they can read it and ask, "Do you know what this is? It's a MICRO-Scope." Get the other one and hold them to your eyes like binoculars. Say, "Do you know what THIS is? It's a periscope!"