There are a few key ways to tell the difference between rubber and silicone. For one, silicone is generally a bit more flexible and smooth than rubber. Additionally, if you put a piece of rubber or silicone in hot water, the rubber will become hard and brittle, while the silicone will remain soft. Finally, if you look at the two materials under a microscope, you'll see that rubber has a more porous surface than silicone.