A few rough edges
Most people may go their whole lives being completely unaware of the fact that dimes and quarters have rough edges while nickels and pennies are smooth. It’s not just a matter of aesthetics, either. These edges were once incredibly important. Coins were once cast from different metals, and its weight represented its true value. As you might guess, judging weight by hand alone is pretty hard, and lots of people realized this too. They would shave off bits of silver to sell off while still retaining the original coin for use. The ridges were added to catch any coin-shavers. Silver was eventually abandoned, but the ridges retained for historical reasons.