Where do they get their energy?

Where do kids get their energy? Here is Kate, its late and she is still rolling around in her bed shouting, "mama, where are you? what are you doing?", she had a totally busy day, friends came over this afternoon and swam, then she spent a good hour running around and jumping in the jump 'n' slide with two little boys who also seem to have endless reserves of energy. For those of you who don't know it is a rare day that Kate ever takes a nap so how she is still awake and where the energy comes from I am perplexed, especially as I need some of it!! Oh to be three again!!!