If anyone wants to know the answer to that question, Richard Feynman explains it wonderfully: (He starts talking about trees at 2:00, but his explanation of fire is equally elegant!) http://www.youtube.com/watch?v=N1pIYI5JQLE

My first instinct was to say that a tree's mass comes from dead stars. Imagining how it got from those stars to the air that eventually becomes the tree would keep you busy for a while :D