Trick? Or Treat? Or...?

My new favorite thing in lab is explaining American customs to Lili.
Case in point from the other day:

Lili: So, if someone comes to my door, they say trick or treat?
Me: Yes
Lili: And, then I will give them the candy?
Me: Mm hmm
Lili: Do I say treat when I give them the candy?
Me: You don't have to.
Lili: So I say trick?
Me: No, don't say trick.
Lili: Oh, it is not a question?
Me: No, it's just a stupid American saying. It doesn't really mean anything anymore.
Lili: But can I say treat and give them candy?
Me: Yes, yes you can.
Lili: Do they get to pick what candy?
Me: If you want to be nice. Or you can give it out as you wish.
Lili: Ok. I will do that.