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Energy is what makes the world go round. Literally. Every neuron that sparks in your brain, every electron that fires down a wire, every molecule burning in a fire, carries with it a sort of momentum that it passes on like a baton in a complex relay race. The batons are flooding in all directions all around us and across the universe - they are energy and we have learned how to harness them.energy pyramid
The actual word "Energy" is a much abused term nowadays - because energy is used to represent such a disparate range of phenomena from heat to light to speed to weight, and because it seems to be able to change forms so readily, it is cannon fodder for pseudo-scientific and spiritual interpretation. However, you will be pleased to hear that it actually has a very clear (and consistent) nature.
I like to think of energy being a bit like money - it is a sort of currency that can be traded. It takes on various forms (dollars/pounds/Swiss francs) and can be eventually cashed in to achieve something. However, just like money, once spent, it does not vanish. It simply moves on a new chapter in its life and may be reused indefinitely.energy pyramid
To illustrate the point, let's follow a 'unit of energy' through a visit to planet Earth to see what I mean. The [number] shows every time it changes currency (see the key below).energy pyramid
The energy in our story starts off tied up in hydrogen atoms in the sun . Suddenly, due to the immense pressure and heat, the nuclei of several atoms react to form a brand new helium atom, and a burst of radiation is released. The radiation smashes into other nearby atoms heating them up so hot  that they glow, sending light  off into space. Several minutes pass in silence before the light bursts through the atmosphere and plunges down to the rainforest hitting a leaf. In the leaf the burst of power smashes a molecule of carbon dioxide and helps free the carbon to make food for the plant . The plant may be eaten (giving food 'Calories'), or may fall to the ground and settle and age for millions of years turning perhaps to coal. That coal may be dug up and burned to give heat  in a power station, boiling water to supply compressed steam  that may drive a turbine  which may be used to generate electricity  which we may then use in our homes to heat/light/move/cook or perhaps to recharge our mobile phone . That energy will then be used to transmit microwaves when you make a call  which will mostly dissipate into the environment heating it (very) slightly . Eventually the warmed earth radiates  this excess of heat off into the void where perhaps it will have another life...