Popcorn in a Paper Sack

I have heard you could do this: Make popcorn in your microwave just by using a paper sack.
Yet, I always was afraid: afraid that the paper sack would burst into flames. Then a friend of mine told me her husband does it almost every night. No oil needed ...just the kernels and a lunch sack. I had to see if there was anything on youtube about this. Yes! The gal did hers for 5 minutes, which seemed too long to me. And she stapled the bag shut before microwaving it. (It did not spark, by the way.)

I read in a magazine that 1/4 cup kernels in the sack is fine, then fold the open end of the sack a few times nice and tight...and microwave 2 minutes. That seemed like not enough time.
After experimenting, I have found that a scant 1/4 cup ( take a tablespoon of kernels off the top of a full 1/4 cup for about 3:00 or 3:07 minutes worked best!
Dump contents of paper sack into bowl and add melted butter (of course!) and a dash of salt