Mister Brown and Squeaks are making bread, and they're going to need a tiny little helper to get the job done: a fungus called yeast! Disciplinary Core Idea: PS1.B: Chemical Reactions - Heating or cooling a substance may cause changes that can be observed. Sometimes these changes are reversible, and sometimes they are not (2-PS1-4). Performance Expectation: 2-PS1-4. Construct an argument with evidence that some changes caused by heating or cooling can be reversed and some cannot.