How does Halloween Trick work? (activity, play, children, kids)

[aidxen]

A kid comes to my door and says trick or treat.
Being non american and not familiar with this activity,
I understand I can say Treat and give them a lollie.

What is the trick part of it.?

[Kathleen1971]

The trick is if you open the door and don't give them a treat lol.

[kahncss]

It's just all done in fun. It's a saying/request by children on Halloween. As they pass from door to door asking for goodies and threatening to play tricks on those who refuse. I think most kids now just say it w/o any thought to it's meaning.

[Katarina Witt]

The origin of Trick or Treat is that you give them a treat, or they pull a trick on you, e.g. knock over your outhouse (long, long ago), soap your windows, TP your house, etc.

As a twist on this, one year when my DD went in a gymnast "costume" (she was a gymnast IRL), someone who knew her asked her to do a trick. She did a cartwheel.