Why do biscuits go soft when you leave them out?

The answer

As a general rule, sugar and many everyday materials are hygroscopic: they take in water from moist air and become damp. The short uses this as its bonus clue: biscuits go soft when they are left out because they soak up moisture from the air, which is why the jar has a lid. It came at the end of a made-up case from Matter of Record Files, in which the last biscuit had vanished. Inspector Fig followed a trail of crumbs past the table and the fridge to the cat, then on down the hall to the mirror, where he found crumbs all down his own coat. He had eaten the biscuit at breakfast, and forgotten.