Gases. Variable volume and variable shape make them the neatness state of matter. A lot of early understanding of chemistry was established through the study of gases. Great scientists like Dalton, Boyle, Avogadro, Lavoisier, etc. discovered amazing things about them.
Now is your chance. Let's start with the basics of Kinetic Molecular Theory.
DeWitt on KMT (first of two): youtu.be/fIMdIMACyN4
Bum Bum Bum Bah Dada Dum?: youtu.be/SJCTgtDU-74