Aerosol spray is a system used in certain containers, like spray cans. The contents of the can, like color or deodorant are pressurized. The can has a valve. When this valve is opened, the liquid is forced out. It will emerge as an aerosol or mist. Apart from the liquid, there is another substance in the container, usually called propellant. When the valve is opened, some propellant will be forced inside the container. That way the pressure in the container is kept constant.