How many times have you watched your kids run laps around the house, jump up and down, climb the sofa like it was Mt. Everest and asked yourself "where do they get the energy?" And how many times have you had to carry your oh-so-tired 40 pound kid out of the Christmas Tree farm, the museum, shopping mall, etc. because they just couldn't walk another step themselves? (While silently cursing yourself for not bringing the stroller, of course.)
It makes me wonder just how that energy comes and goes and I've come to the conclusion that it is parentally driven and it counter-corresponds directly to the amount of energy you want your child to have and expend at any given moment.
For example, if you want your daughter to sit down and have dinner, she will be running up and down the stairs faster and faster. However, if you are at and apple picking farm all the way down at the bottom of a big hill and carrying a bag filled with apples, at the farthest possible point from your car, that same daughter will be too tired and ask to be carried.
It's one possible theory.