I get good mileage out of placing a glass stirring rod into a container
of Wesson canola oil. The plastic cap on the bottle effectively hides
the rod from above, and though the rod is faintly visible from the side,
other reflections tend to diminish the likelihood of its being noticed.
From beneath, the end of the rod is quite apparent, but since I pass the
bottle around the class without hinting that something other than oil
might be inside it, nobody has yet seemed interested in a close
examination.
The "story" that I tell while the bottle is circulating has to do with
index of refraction, of course, and once the bottle has made the rounds
and the story has ended, I pull the rod out with a pair of long tweezers
and astound the masses. They always want another look, and can usually
find the rod (though still not without difficulty).