Don't get the logos on these two pieces of merchandise mixed up. One of them represents Indiana University; the other symbol generically represents what profession/field of knowledge?
As seen above, quarters have ridges around the rim. So do dimes, half dollars, and Susan B. Anthony dollar coins (but not pennies or nickels). What was the original reason for this design feature?
BONUS: In 2000, Susan B. Anthony dollars were replaced by Sacagawea dollars, which don't have ridges. What was the reason for not giving them ridges?
Quarters used to contain silver (in fact, at one time their precious metal content was the basis for their value; it's not a coincidence that a quarter weighs exactly as much as 2.5 dimes!). "Coin clippers" would cheat by shaving precious metal off the edges of coins and then spending them at their face value. The ridges on quarters prevent that practice by making it obvious if a coin has been shaved. Pennies and nickels didn't need ridges because they never had much intrinsic value.
Eventually, coin clipping ceased to be a problem. You might think this is because of inflation, but silver has held its value pretty well; it's just that U. S. coins from 1965 onward don't contain any! You'll almost never see a pre-1965 quarter in circulation, because they're worth about $3 melted down.
Quarters still have their traditional ridges, but when the government introduced the Sacagawea dollar in 2000, they gave it a smooth edge to help the visually impaired distinguish it from a quarter (and gave it a different color for the rest of us). They hoped resolving one of the perennial complaints about Susan B. Anthony dollars would encourage people to use dollar coins, but to no avail; the Sacagawea dollar is just as unpopular as its predecessor.
Cheers to these folks who answered one or both questions correctly: Hazel, Peter M., Zane, Harper, and Leo S. (who tells me that the ridges were Isaac Newton's idea). And thanks to Aditi for informing me that Charlotte can use a quarter to cut open a watermelon. 🍉