Why do old biscuits go soft but bread goes hard?

Why does a biscuit that is left in the open overnight become soft by the morning, but a bread roll left out for the same time becomes so hard that one could knock someone out with it?

Why does a biscuit that is left in the open overnight become soft by the morning, but a bread roll left out for the same time becomes so hard that one could knock someone out with it?

Biscuits contain much more sugar and salt than bread rolls. The finely milled sugar and salt absorb moisture from the atmosphere. The bread roll, on the other hand, contains little salt or sugar and its main ingredient, flour, is indifferent to moisture in the air. So, because of their different make-up, one attracts water and the other doesn't.

- Why Don't Penguins' Feet Freeze? edited by Mike O'Hare

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