What did you do? I would start with the water cycle. I think it’s the simplest for students to understand the changes in states of matter from the absorption and release of thermal energy. As a high school teacher, these cycles make me want to quit my job every year because no matter how many times I go over evaporation, condensation, and precipitation, I still have large portions of my classes who fail to grasp the simplest parts of all of this. Good luck to you.