Popcorn is one of those extra expenses that go with movie-going. How’s a fellah gonna watch Captain America save the nation without a large tub of salty, buttery popcorn to get him through all those action scenes?By the way, that buttery flavor we love on our popcorn likely isn’t butter. Medical studies say it’s harmful, not so much to those of us who eat it, but to popcorn-factory employees and concession-stand workers who inhale that aroma for hours at a time.According to About.com, that buttery flavor is a chemical called diacetyl.