Great question, and you’re absolutely right – it is something to do with the molecules! When you use an iron, it heats up the fabric of the shirt, which loosens the bonds in the molecules that make up the fabric. The weight of the iron then forms the molecule into a new shape, which sets when the shirt cools down again. It’s kind of like when you heat up a bar of chocolate – it makes it malleable, and you can bend it into a different shape. If you let it cool down again, it sets that way!