Put 1/3 cup uncooked popcorn kernels into a plain brown paper lunch bag. Add 1/2 tbsp butter broken into small pieces on top of the kernels.

Fold the top of the bag down twice to close it, leaving a little room for steam to expand (do not seal airtight).

Place the bag in the microwave with the folded side up.

Use the "popcorn" setting; if your microwave has no popcorn setting, microwave on high and stop when popping slows to about 2 seconds between pops (typically 1½–3 minutes depending on your microwave).

When popping slows, stop the microwave and let the bag stand for 1 minute to allow any remaining kernels to pop.

Carefully open the bag away from your face to avoid hot steam.

Season with salt and serve.