Not sure if any spray paints still have this, but why did some older spray paint cans had a marble, or such, in them? And, why did others not need this?
It significantly assists in the mixing of the paint as you shake the can prior to spraying. (And it’s not just older cans - I bought one last week that had this.)
I can say with some authority that when the can stops spraying it doesn’t mean there’s no pressure. Used a can opener to remove the marble. My folks weren’t happy with the red painting the wall. :eek:
The ones I opened a few years ago had a piece of steel shot, not a marble, in them. If you want to get one out without too much mess, spray the can until it stops, then put it in a freezer for an hour or two. Take it outside, point it away from you, and use an old-fashioned can opener to poke a hole in the bottom.
You know that rattle when you shake up a spray paint can? That’s a kid’s tooth
Good advice. Where were you in 1955 when it would have helped.
Or in 1990 when I drilled a hole in the side of an old deodorant can. “Hey, it’s empty” drill drill drill pshhhhhh. “Nope, not empty”
Still not sure what I thought I was going to accomplish by drilling the hole though.
(bolding mine)
To see WHAT was INSIDE, of course! 
WAG: the little ball is there so you can hear if you shaken the can enough? Anyway that’s what I use it for.
This is a common misconception. In fact the “marble” or “pea” is the seed of the paint tree. Although these days they grow almost exclusively in ordered rows on factory farms there was a time when the paint tree was a common sight in North America. That which we call “paint” is actually the mesocarp, or fleshy portion, of the paint fruit, and due to the chemistry of the fruit’s development it is naturally under pressure. Humans have long drained the fruits for their paint, but it was not until 1949 that Bonnie Seymour developed the technology to lop off the stem and attach a valve to the fruit without losing pressure. Thus “spray paint” was born.
Unfortunately, due to poor husbandry our current cultivated population of paint trees suffers from an artificially-limited genetic base, and many paint shades that were once common have become rare or extinct. Luckily the paint tree is quite prone to mutation, and unculled populations tend to manifest novel colors within three generations. In order to return this majestic species one would do well to encourage its wild proliferation. As the paint tree easily finds root with even minimal nutrients and light this is best accomplished by discarding empty “spray paint” in fields, by the side of the road, in public gardens, or anywhere else they might germinate.
One good tip that I learned years ago isthat you can duplicate this for small bottles of paint (like model paint bottles, or car touch-up paint, or fabric paint, or whatever).
if you put a small ball bearing, or similar piece of metal (a small nut, for instance) in a bottle of said paint, you can then use it to mix the paint by simply shaking the bottle prior to opening it. It’s better than stirring the paint up with your brush, which usually gets the brush wetted much farther up than you’d like it.*
*And, of course, this explains WHY the spray paint cans have such a ball when other paint cans don’t – you can’t open the pressurized spray paint can to stir up the paint they way you can a container of wall paint.
How old were you at the time? If you were in your teens, then probably you weren’t thinking at all. 
You gave me shudders saying that. Never stir the paint with a brush. Never get paint into the ferrule of the brush. We called that “studenting” a brush, as in that’s why I only gave inexperienced students cheap brushes lest you be left with nothing but bricks in the shape of a paint brush.
The one time I opened a can I found a small rivet that was being used as the “marble”.
They grow like pearls inside of paint cans. Of course in modern paint cans the marble is almost always cultured, natural paint marbles are very rare.
As a kid with a bb gun I did the following:
Took a full spray-paint can from the garage. Took a large plastic flowerpot from the garden. Placed the spray-paint on the grass in the backyard. Covered it with the flowerpot. Shot through the plastic flowerpot several times until I hit the spray-paint can.
So then we had a large, bright red circle on the backyard grass, and a flowerpot with a few tiny holes and a bright-red interior.
Mission accomplished.
You might still get ‘colorized’, doing it that way. I have.
I now just shoot them with my pellet gun from about 10 or 20 yards away. Then I cut them open. Who cares if the tree you leaned it against (or the grass in front of the can) turns red?
This reminds me of the time when I had a can of deodorant which had a rubber plug in the bottom (the only time I’ve ever seen this on an aerosol can) and decided to pull it out. Not actually as bad as you might think (I had it pointing up); a big blast of pressurized gas and the can got very cold; I was able to put the plug back in without losing much of the contents (and it still worked; the liquid boils away until it cools down, butane boils at 30F and propane at -43F, after which the rate of evaporation decreases). I also unscrewed the value on a disposable lighter before, same thing (turning it upside down causes the butane to blast out).
I’ve always wondered why aerosol cans needed warnings about “do not puncture or incinerate”. Now I know why.
That’s what I was told, but how does the marble know???