photos taken near Central Square in Cambridge, MAToday I learned about that little ball inside a spray paint can. Spray paint allows you to spread paint evenly on a surface. Inside the can is the paint, aerosol propellant, and the little ball. When you shake up the can, the tiny ball mixes the paint so that there is an even coat when yous spray and the mixing of the propellant and the paint creates the pressure that is released when you open the valve to spray the paint. This is why shaking the can is VERY important to create a nice smooth and even coat of paint on the wall. So when you start painting with your spray can make sure to shake it up really well so that you can hear the little ball mixing it all up! Also paint outside or where there is good ventilation since spraying creates small paint particles that you can breath in which would be bad for you lungs.