Popcorn
Porcorn is a practical movie snack. First, it stops you from having to go to the toilet during the movie (even if you've drunk half a large Coke). The salt in the popcorn absorbs the excess water in your body, as it does in potato chips. Second, unlike potato chips, popcorn is a quiet food to eat during a movie, not making any loud, obnoxious crunchy movies to obscure the dialogue.