There are two types of energy: Potential energy (stored energy) and kinetic energy (moving energy). All forms of energy are stored in different ways in the energy sources that we use every day. These sources are divided into two groups, renewable energy and non-renewable energy. Renewable and non-renewable energy can be converted into secondary energy sources like electricity and hydrogen. Secondary sources of energy are used to store, move, and deliver energy in easily usable form. We have to use another energy source to make electricity or hydrogen, e.g. coals.

Energy is measured in joules (J). The work done, or energy used, during a certain time is called power. Power is measured in watts (W). A watt is the use of one joule of energy for one second. A watt is a small amount of energy, thus we measure power in units of 1000 watts for one hour, kilowatts for one hour. 

Non-renewable energy is an energy source that we are using up and cannot recreate in a short period of time. We get most of our energy from non-renewable energy sources, which includes the fossil fuels (oil, natural gas and coal). They are called fossil fuels, because they were formed over millions of years by the action of heat from the earth's core and pressure from rock and soil on the remains (or ‘fossils’ ) of dead plants and animals.  Examples of non-renewable energy sources are coal, petroleum, natural gas, propane, oil and uranium.

Renewable energy is an energy source that can be replenished in a short period of time. Renewable energy sources are solar energy (sun), wind energy, geothermal energy (from inside the earth), biomass (from plants) and hydropower (water). We use renewable energy sources mainly to make electricity. Also referred to as sustainable energy, which means the sources of energy should be replenishable within a lifetime and should not cause any longterm pollution or damage to the environment.

Polar bears waste lots of their energy and it could be a problem - We thought polar bears had neat tricks for conserving energy in lean periods, but it turns out they are not that thrifty, which could cause them trouble in the future
Deadly solar flares may have helped seed life on Mars and beyond - High-energy particles that can strip away planetary atmospheres and cause biological damage might also forge the complex organic molecules that give rise to life

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