Why does cookies retain their crispness only when kept in air-tight containers?

Cookies retain their crispness in airtight containers because of a simple concept: moisture control.

Here's why:

* Cookies are porous: They have tiny spaces within their structure that can absorb moisture from the air.

* Moisture makes them soft: When cookies absorb moisture, the starch molecules in the dough swell, causing the cookie to become soft and chewy.

* Airtight containers prevent moisture exchange: Airtight containers create a barrier that prevents outside moisture from entering and the moisture inside the cookie from escaping. This keeps the cookies in a dry environment, preserving their crispness.

Why exposed cookies become soft:

* Humidity: The air around us contains varying levels of moisture (humidity). Cookies exposed to the air will absorb this moisture, especially if the air is humid.

* Other foods: Even in a pantry, cookies can be exposed to moisture from other foods, like fruits and vegetables.

Tips for keeping cookies crisp:

* Use airtight containers: Choose containers that seal tightly and don't have any leaks.

* Store in a cool, dry place: Avoid storing cookies in areas that are hot or humid, like near a stove or in direct sunlight.

* Don't overcrowd the container: Give cookies some breathing room so they don't get crushed and absorb moisture from each other.

* Consider freezing: Freezing is the ultimate way to preserve crispness. Simply freeze cookies in an airtight container, and they'll stay crunchy for months.