- a bag of popcorn
Ask the class if the would like some popcorn.
After someone volunteers take out a piece of popcorn chew it up and spit it back into the bag. Shake it up then offer it to the person who wanted it. (they should no longer want it.) Then simply say, "What, it is only one little piece."
Small things can contaminate something that is otherwise good, like a movie, book, song or tv show.