The Baseball
Cork, wool, leather and 108 stitches, and small changes to it have moved league offense more than any rule ever has.
What it actually is. A baseball has a cork and rubber core, wound with layers of wool and cotton yarn, covered in two pieces of cowhide stitched together with 108 double stitches in red thread. It weighs between five and five and a quarter ounces and measures about nine inches around. Every ball used in a major league game is rubbed with a specific mud before it is played with.
What somebody who knows the game gets out of it. The ball is the most powerful and least visible lever in baseball. Small changes to seam height and to the coefficient of restitution change how far a struck ball carries and how much a pitched ball breaks, and the effects are league wide and instant. The 2019 season, when a record number of home runs were hit, and the seasons on either side of it are the clearest modern case, and the dead ball era and the sudden offense of 1920 are the historical one. When you see league home run rate jump in a single season with no rule change, the ball is the first suspect.
The numbers worth knowing. 5 to 5.25 ounces, 9 to 9.25 inches around, 108 stitches. A major league game uses roughly 100 to 120 baseballs. A change in drag of a few percent is worth a few hundred home runs across a season.
What people get wrong about it. Seam height matters as much as the core. A slightly lower seam reduces air resistance, so the ball carries further and breaking pitches move less, which is why pitchers complain about grip in exactly the seasons when home runs spike. Those two complaints are the same physical fact seen from opposite ends.
At Little League and travel ball. Youth balls differ by age: softer, safety rated balls at the youngest levels, then standard balls with different compression ratings. Compression, usually stamped on the box, changes how far the ball travels and how much it hurts, and leagues specify it. It is one of the equipment rules parents most often get wrong.
Batter's Eye: look up anything in baseball