Some, but not all, U.S. coins have ridges on their side, but have you ever wondered why?
Back in the 1700's, coins were made out of precious metals like gold and silver much more often than they are today. When ridges weren't a part of the coin's build, some people filed down excess metal from the edge of the coin and collected it to sell it for a lump sum profit.
When the government started catching onto this behavior, they not only began changing the metallic composition of our coins to put a stop to this devious behavior, but they also added ridges, which make it far easier to tell whether or not a coin has been tampered with.
In addition to keeping people from filing off excess metal when they're not supposed to, these ridges help prevent coin duplication by making them more difficult to reproduce.
Today, since precious metals are barely used in coins, the ridges can help visually-impaired individuals to more easily distinguish between coins of similar size.