What makes foods get harder or softer as they age?

It's time for Shower Thoughts, our weekly attempt to answer the questions that get stuck in your head and refuse to leave. Tonight's one is this: Why does something like a loaf of bread go hard and stale as it gets older, while fruit goes the other way, getting softer and softer? What's actually happening to these foods at a structural level before they end up in our mouths? Well, to help us get to the bottom of it, Emile is joined on the line now by food physicist Dr Sofia Rodrigues from the University of Waikato.