I went to a random page in my physics textbook and came across the First Law of Thermodynamics topic. I am intrigued by the simplicity of the mathematical statement of this law, which is dU = dQ - dW. (dU is the small change in internal energy, dQ is the small change in heat, and dW is the small amount of work done by the system). The very first verbal description of the First Law of Thermodynamics I read in the physics textbook is that this Law is a principle of conservation of energy, that energy cannot be destroyed and created but converted from one form to another. My question is how does that description (definition?) translate to the mathematical statement dU = dQ -dW.