Spray cans produce an aerosol, the technical term for a very fine spray. They do this by means of a pressurized propellant, which is a liquid that boils at everyday temperatures. Inside the can, a layer of gaseous pressure increased, and eventually it becomes so high that boiling stops. when the nozzle is pressed, the gas pressure forces the product up the tube in the can and out of the nozzle in a spray or foam. The propellant may emerge as well but, now under less pressure, it immediately evaporates. First patented in the US in 1941, aeroso ....