Boyle's law, the principle that the pressure on a gas is inversely proportional to its volume at constant temperatures, is demonstrable with everything from balloons to soda cans to SCUBA gear. Aerosol cans and syringes both rely on Boyle's law in order to perform their functions as well.
In an aerosol can, the contents are mixed with a gas under pressure high enough to render the gas into a liquid. When the nozzle is opened, however, this reduces the pressure suddenly, allowing the gas to expand. As it does so, it forces its way out of the nozzle, carrying the contents with it. The same effect occurs when a pressurized soda can is opened, allowing pressurized carbon dioxide to form bubbles in the beverage.
Drawing blood with a syringe uses Boyle's law in a different way. When the plunger of the syringe is pulled back, it increases the volume of the chamber inside the syringe. This reduces the pressure, creating a vacuum effect. This draws blood into the chamber through the needle.
Divers must keep Boyle's law in mind at all times. When a diver descends, the pressure around him increases. This forces nitrogen into solution inside his bloodstream. If he ascends too rapidly, the dissolved nitrogen expands suddenly, forming bubbles. This causes the painful and potentially life-threatening condition known as the bends, and only returning to a pressurized environment can reverse it.