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Renewable energy sources are basically anything that doesn't expire once used. For example, lets look at a coal fire to illustrate this point
To start the fire you need to use paper, once it's burnt that's it... it's gone forever.
Next you'd use kindling to get the flames a little stronger, but again once the kindling has burned you'll never see it again.
So on with the coal to get a roaring fire. With every load of coal that goes on the fire, once used it is unusable again. This is NOT a renewable energy source, once burned it's gone for good. This coal has to be mined from the earth, so we are actually taking resources out of our ground and burning them. You can see how quickly this process could lead to shortages if all the houses in the world were still relient on coal for their fires.
So instead we look to renewable energy sources that won't run out. Sun, water and wind which can all be harnessed in some way to generate our energy needs. Go here to read more about this fascinating subject - renewable energy sources