Mason Jars: The Two-Part LidWhy the ring and the disc must be separate

From the show

Why it looks like that

Unlike commercial jars, home canning jars use a cumbersome two-piece metal lid. This design solves a critical thermodynamic problem: venting. As the jar boils, expanding air needs to escape to create a vacuum. The separate ring acts as a flexible spring, holding the lid down while allowing it to lift just enough to "burp" out air without letting water in.