Some grinders have two or three compartments instead of just one, with fine screens separating the bottom compartments from the ones above, thus allowing the marijuana trichomes, also called kief, to be collected separately.

Why do some grinders have screens?

Your grinder most likely has a screen because it has a place for pollen to fall into from your herbs while you're grinding. It's possible for some of the pollen to get stuck. To fix this, put a penny in the pollen chamber so that when you're shaking the grinder around, it should bang the pollen into place.

What is the purpose of a 5 piece grinder?

The kief catcher, located at the bottom of the grinder, makes the 5-piece grinder the best grinder for kief. This grinder conveniently collects all the kief that falls off your bud when it is being ground. Your bud chamber will have a screen that allows trichomes to fall through to the catcher.