A period history question from the movie Meet Me in St. Louis?

My wife and I just watched a period holiday movie titled, Meet Me in St. Louis. The movie takes place in 1903. In the movie during Halloween the mother gives the 5 year old daughter a bag of flour and tells her to throw the flour in peoples face after they answer the door. Along with that the neighborhood children proceed to build a huge fire in the middle of the streets with no adult supervision. I sort of get the barn fire thing as something more excepted back then because times were so different but I have no clue what throwing the flour in peoples face is all about. I have thought and rethought this and have come up with no ideas. I understand why such a tradition is no longer practiced as there would be many upset adults by today's standards. Is there anyone out there that understands the meaning of this flour throwing? Thank you for your time.