 stuff that comes off of willow trees, weeping willow trees. You've got that kind of grass stuff. Anything that gets in there will start plugging that vent up. That is your air intake for all the air that comes into your vehicle's HVAC system. Now, it does a couple other vents to make sure that there's ventilation to the vehicle when you're driving down the road so you can't be asphyxiated. But for your heating and air conditioning system, everything comes through that vent at the base of the windshield normally on the passenger side.