Tell the students you're going to release a critter into the Petri dish. Carefully, squeeze a few small drips of glue into the dish. Make it look like you're pouring something out of the bag - don't let them see the tube of glue. Play it up!
After a bit, add some pencil shavings to the dish - the critter will "eat" them.
What happens if you add a drop of soapy water or dip a bar of soap into the water?
And then for the big question: are the critters alive? What evidence do you have? How can you explain it?