Energy

Energy is one of the most powerful concepts in physics and can often be a simple and basic way to understand a phenomenon because of the following:

1. Damn near everything can be expressed in terms of energy, so it can be used as a currency to compare and equate very different things. 
2. The total amount of energy never changes. If one thing loses energy, something else must have gained it. 

Consider a classic example:
it takes energy to lift a ball some height against gravity, and the amount of energy required can be calculated (e = m * g * h).
It also takes energy to change something’s velocity (e = 1/2 * m * v²)

So, knowing these two things you can both determine how high a moving thing will rise before coming to rest and determine how fast a thing will be moving once it falls a certain distance.

Suppose that falling ball bounces. You’ll see that it does not rise quite to the height from which it fell. This way we know that there’s energy to be found somewhere. What you’ll find is that the ball and the surface it bounced off of both heated up a very small amount. (that's why this works) Maybe the surface dented, that took energy. There’s also the sound of the impact and some stirring of the air. These all have an energy equivalence and together account for the ball eventually coming to a rest.

So when you walk, don’t scuff your feet. You are using extra energy to generate the scuffing sound and to wear off bits of sole.