Converting energy
Every action requires energy to be converted for the action to take place. This conversion is governed by the following two main rules: the total amount of energy in the universe is constant and never changes; energy can be converted from one form to another, but can neither be created or destroyed.
Each time energy is converted, a little heat is released. For example, when petrol is converted into mechanical motion in a car, part of the energy is converted into heat rather than into movement. We call this a "loss" of energy (i.e. the amount of energy at the outset = the amount of energy produced + heat).
Only when energy is converted for the purpose of producing heat is there no "loss" (for example when electricity is converted into heat by a cooker).
