Automatic vent mechanism. They allow for automatic venting of the greenhouse. The principle is: A piston is filled with a wax that expands in the heat, opening the windows and shrinks again in cooler weather closing the windows. The system works best if you have a minimum of two vents. One in the roof and one in the side. The roof one will allow hot air out and the side vent, the cool air in. If these two openings are at opposite ends, it will allow for the greatest circulation of air.