I hope not to sound too much like Morpheus if I tell you that the human body is an amazing source of energy. As long as given food, water and oxygen, it will burn hydrocarbon chains to keep a steady temperature of some 37 ºC in a wide range of situations, activities and external weather conditions.
We use that energy in every interaction with the world. When I push these plastic keys against their springs, so that they can come in touch with the other contact of their respective switches and send an electric signal to the CPU (who will process it and translate it to that precise letter shown on my screen), I’m using the body energy to produce some work on them, which overcomes the stiffness of the springs and produces an elastic deformation that will be recovered the moment I raise my finger from them, so I can press them again and make that same letter appear onscreen if I want to. In nearly every other action we do, the same scheme applies: we transfer some of our body energy to some other object of the external world (e.g., to the floor when we walk, to the door handle when we open it, etc) by means of work. But we also exchange heat.