What is why are there marbles in spray cans?

There are no marbles in spray cans. However, some spray paint cans use a mixing ball made of metal to help blend the paint before use. The ball rattles around inside the can, helping to mix the paint and remove any settle pigments in the process. This can create a shaking sound that may seem like there are marbles inside the spray can. The use of mixing balls in spray cans helps to ensure that the final product is evenly distributed and applied uniformly.