 Alright, so it turns out when water boils, it turns into steam. Steam is a gaseous state of water and it is a steam which is moving the paper that is kept on top of the container. So we can say that steam has the ability to do work. But where did steam come from? Well, it came from the heat that we are providing to the water, right? So we can also say that heat is doing the work of moving the paper up and down. And we know that energy is really the ability to do work. So heat can be called as a type of energy. This ability of heat or steam to do work is the principle behind steam engine. In steam engine, water is heated in huge boilers and then steam is generated because of that. That steam helps push pistons or cylinders which can help move the train forward. So the principle that is responsible for these steam engines, these trains to move is the same principle that you can arrive at in the comfort of your home or in your classroom. And that shows that science can be done anywhere and that it's also pretty awesome.