Few things ruin cookies like hardness. When you bake fresh cookies, you expect them soft and chewy when you eat them. Keep your cookies soft with an item nearly everyone keeps readily on hand: basic sliced white bread. White bread imparts moisture in storage containers and keeps home-baked and store-bought cookies soft and fresh for days. Most cookie jars are for decoration, not preservation, and don't seal tightly when closed. Airtight storage containers, on the other hand, are made for long-term storage and provide the best staleness protection.
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Place the cookies into an airtight container. Place a slice of white bread to the side or on top of the cookies.
Store the cookies in a dry cupboard overnight.
Remove the bread and discard it. The bread will be hard, the cookies soft. Repeat if needed.