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We found the answer to your question in the "Aerosol Can" entry of HowStuffWorks.Com. Today's aerosol can varies only slightly from its original 1931 patent. This is good news for the estate of Norwegian inventor Eric Rotheim. Every aerosol can has two important elements: the product and the propellant. The product in a can of spray paint is simply the paint itself. The propellant, however, is the material that shoots it out of the can. The propellant is a liquefied gas. At room temperature it's a gas, but it's forced into the can at a high pressure, causing it to liquefy. Pressing down on the can nozzle releases that pressure, and some of the propellant
reverts to gas. This shoots the paint out. HowStuffWorks offers an excellent diagram of the process. So where does that rattling sound come from? A metal ball bearing helps mix, or "agitate," the product and propellant. When the two liquids are mixed equally, the paint shoots out of the can nozzle in a fine mist, rather than in clumps.
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