"Why do we say Trick-or-Treat?"

Where does that come from?
As you can see from the pictures I put above this post, tonight was pumpkin carving night. We had a really fun time and even managed to stay clean for the most part. The kids loved it, which is the most important part, but I was posed a very perplexing question during all the fun.
"Daddy, why do we say 'Trick or Treat' when we go Trick-or-Treating?" my little girl asked me. I honestly had no earthly idea. So, do I come up with some BS answer and sound like I know what I'm talking about? Or, do I take the chickenshit way out and tell her I don't know? I took the middle ground and said, "It's just a tradition Sweetie." As you may be able to predict the next thing out of her mouth was "What's a tradition?" Holy crap what does this little girl think? She must think that because I'm her daddy that I have all the answers and am the smartest man alive. Where would she ever get such an idea? Oh yeah, I've told her that many times over. Man that gets me in trouble sometimes. However, I was lucky tonight because after she asked what a tradition was my wife "saved" me, even though she hadn't heard any of the conversation and didn't even know she had my back, by plopping the pumpkins onto the table. That distracted the little girl from the fifth degree she was applying to me long enough for me to escape to the living room for the camera.
But, I figured that if I was going to tell my kids that I was brilliant and knew everything, I should at least know WHERE to find the answers. At least that way I won't look like a complete idiot. The internet has all the answers I'll ever need. That is why I looked up the origin of Trick-or-Treating so I could surprise my daughter with the answer tomorrow.

What I found is on wikipedia and I put a link to it here.http://en.wikipedia.org/wiki/Trick-or-treating
I hope you find it useful because I know I am going to study up so that in the morning I can surprise and amaze my four year old with all of the neat facts about Trick-or-Treating.
Now my problem is that she's going to get older at some point and ask me about sex, and I'm still not going to know the answers right off the top of my head. AND, I sure as hell am not sending her to the internet for the answers to that question and lead her right to internet porn! That's why I am practicing my new standard answer to any questions I do not know the answer to.
"Go ask your Mom."