First, picture a cow’s hind leg—yes, the muscular, hard-working part that does all the walking and mooing. The top round comes from the inner thigh, which is a slightly more tender area because it does less work. Think of it as the "office worker" of the leg muscles—still a bit tough, but not completely inflexible.
The bottom round, on the other hand, is from the outer thigh, which handles a lot more of the heavy lifting. It’s the one who runs the marathons while the top round just jogs to the water cooler. This makes the bottom round tougher, leaner, and more likely to dry out if you look at it wrong.
