You might think coins have 2 sides but they actually have 3 when it comes to coin appraisal. The peripheral edges around a coin constitute the 3rd side. Mintmarks can tell you a lot about a coin's origin too.
If there's no mintmark for coins before 1982 or a P on it after 1982, it was struck in Philadelphia. The minting process isn't perfect, so there are often variations and tiny errors or differences in specific sets of coins. These error coins often go out of circulation, resulting in a very rare coin.
Anatomy of a coin
“There are actually 3 and not 2 sides to U.S. coins.”