Here's why:

* Eggshells are primarily made of calcium carbonate.

* Vinegar is a weak acid (acetic acid).

* Acids react with carbonates to produce carbon dioxide gas, water, and a salt.

When you place an eggshell in vinegar, the acetic acid in the vinegar reacts with the calcium carbonate in the eggshell. This reaction produces carbon dioxide gas (which you'll see as bubbles), water, and calcium acetate (a salt). The calcium carbonate in the eggshell dissolves away, leaving a thin, rubbery membrane behind.

It's a fun and simple science experiment to try!