Pop the popcorn, being careful not to burn it. (Because NOTHING tastes worse than burnt popcorn!) Dump the popcorn out of the bag and into a bowl, being sure to remove any kernels that may have slipped through.
In a saucepan, melt the butter over medium heat and slowly add the marshmallows, stirring constantly until melted and smooth. Remove from heat.
Pour the sticky marshmallow goo over the popcorn and mix thoroughly, making sure it's evenly coated. Allow to cool before you try to shape the balls! When it's cool enough to touch, shape into balls with your hands. You can butter your hands or use wax paper if it's too sticky to handle easily.