If I'm in the right place, I'll make them a pour-over with fine ground coffee, then another with medium ground coffee and let them compare. Every time, they've chosen the medium over the fine, and that's because the fine ground coffee has much more overall surface area compared to medium or coarse, making the water flow through it slower, picking up more flavor which may not be ideal depending on your brewing method.

For espresso, use fine

For a pour over, use medium

For a traditional drip, use medium

For a french press use coarse

For cold brew, use coarse