Conversation so far:
why did the tradition of fancy dress for halloween begin
The practice of Halloween Costumes may have originated in a Celtic festival called Samhain, held on 31 October–1 November, to mark the beginning of winter.
how much money is spent on halloween costumes
In 2017 it was estimated that Americans would spend $9.1 billion on Halloween merchandise with $3.4 billion of that being on spend on Halloween costumes.
has any halloween costumes caused any controversy
There have been controversial Halloween costumes over the years. One that sparked enormous controversy well before Halloween 2015 is a Caitlyn Jenner corset costume.
is halloween costumes only for kids
Halloween costumes have tended to be worn mainly by young people, but since the mid-20th century they have been increasingly worn by adults also.

Question: what are some common costumes in the practice
Self-contained rewrite: what are some common halloween costumes