ANSWER:
The first law of thermodynamics states that: "The total energy of an isolated system is neither created nor destroyed, it remains constant."
Although the definition seems very technical and difficult to understand, there are numerous examples in everyday life that apply this thermodynamic principle.
For example:
The conservation of energy in a ball thrown into the air.
There are two types of energy involved in this example: kinetic and potential.
0. A boy throws a ball into the air.
,1. At the moment that he leaves his hands the ball has speed, therefore it has kinetic energy. It hasn't gained height yet, so it has no potential energy.
,2. As it goes up it loses speed and gains height. He loses kinetic energy and gains potential energy.
,3. When it reaches the highest point, it only has potential energy.
,4. Finally it goes back down and the energies are reversed again.
