Where do they get energy?

I have been watching Reagan a lot over the last week. She is just full of energy. She doesn't stop. When we get home, she starts running and really doesn't stop until we say "ok time for bed". It doesn't matter where she's at, she has to run. I aksed Nicki how she was doing on the playground. She said, she likes to run, but hasn't quite figured out the slope in the yard, so she clings to the gate a lot. I can't wait to see her figure that out.