Matter and energy can be neither created nor destroyed, but matter can sometimes be converted into energy, and energy can sometimes be converted to matter.
The energy of stars comes from a slow conversion of matter to energy as light atoms join to make slightly heavier atoms. Stars do this for billions of years.
All change requires energy. Energy is what you need to do work. Energy can be electrical, chemical, or of other sorts. Energy is neither created nor destroyed.
Energy is the cause of all change in all systems, everywhere. Energy can be converted from one form to another. Power is the rate of transfer of energy.
Heat makes changes of state happen: the processes of boiling: condensing: melting, subliming and solidifying all happen when heat is added or taken away.
Energy operates within in accordance with the laws of thermodynamics, and can only be understood, used and manipulated within those assumptions.
Tensioning a bow or any other elastic item increases its potential energy, which is converted back to kinetic energy when it is released again.
Falling weights, expanding gas and burning material all release energy, and can be used to do work. Most machines convert energy from one form to another.
The first form of energy assistance for humans came with the use of draught animals, hence the name of the original unit for work done, the horsepower.
We waste valuable domestic energy by bad housing design and a lack of insulation. This leads to a cost which is not usually borne by the wasters.
In 1834, Benoit Paul Emile Clapeyron presented a formulation of the second law of thermodynamics, giving us for the first time, the notion of entropy.
James Joule spent most of his time experimenting. The joule derives from his studies which showed that heat and mechanical movement are forms of the same thing.
Joule's work proved that heat and movement were both forms of energy, and that was as good as a theoretical proof. It also put a measure on the equivalence.
Joule's predecessors include Plato who saw 'heat and fire' come from 'impact and friction', and Count Rumford, who studied the heat produced in cannon-boring.
Maxwell's demon is a fascinating paradox which implied that there might be a way to beat the laws of thermodynamics and achieve perpetual motion. You can't.
The Maxwell's demon paradox is based on the fact that some gas molecules have more energy than others: if a demon could separate them, this would yield energy.
When James Clerk Maxwell proposed the paradox, he would have known that there must be an answer, but he could not immediately see what it would be.
The answer to Maxwell's demon is that the energy needed to separate the molecules would be greater than the energy that could be obtained from the differences.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsmatterenergy.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.