Show the class the caramel covered onion. Really talk up the "apple". Tell them how delicious it is (they probably won't need much convincing.) Invite a volunteer to eat it and describe to the others in the class how good it is. At this point, depending on the volunteer's nature and how far you want to take the lesson, you can either ask the volunteer to take a big bite into the apple or you could cut it, revealing the onion inside, and then let them off the hook once they realize what it really is.