Melt the two sticks of butter in a large pot (I use my large stock-pot).

Pop one cup of popcorn.  (We have an electric popcorn popper that we found at the thrift store for very cheap.  We buy popcorn in bulk from our food co-op.)

When the butter is melted, pour the bag of marshmallows into the pot with it and stir until the marshmallows melt.

Pour the popped corn into the melted butter and marshmallow mixture and stir until all the popcorn is coated.  Grease hands with butter (this is the part kids like to help with).

When the popcorn is cool enough that you can hold it in your hands without getting burned, form it into balls.  Lay the balls on waxed paper to cool.