Trick or treat???

 It is great fun to dress up in a costume and getting a lot of candy. But I am always a little troubled by the phrase you are supposed to say when you come to each house - "Trick or Treat!?"

 what does that phrase mean? I was giving the people in that house a choice - either give me a treat (candy), or I’ll give you a trick (a mean one). Of course, I will not do something mean to someone who does not give me candy .

So...