Here's why:

* Porous structure: Egg shells are made of calcium carbonate and have a porous structure. These pores allow for the passage of small molecules like salt ions.

* Osmosis: Salt dissolves in water, creating a concentration gradient. Water moves across the egg shell membrane from areas of high water concentration (low salt concentration) to areas of low water concentration (high salt concentration) in an attempt to equalize the salt concentration on both sides. This process is called osmosis.

Practical example: When you soak an egg in salt water, salt diffuses into the egg, and water moves out of the egg, causing it to shrink.

Note: The permeability of the egg shell can be influenced by factors like the age of the egg, the size of the pores, and the temperature.