Running,
Jumping,
Laughing,
Playing ball,
Sweat and dirt don't seem to bother them.
They almost welcome it.
Perpetual motion,
Never-ending energy.
Their heads hit the pillow at night,
Exhausted,
But, fighting sleep.

(I wrote this many years ago when my son, who is now 20, was about 6. I was sitting in the bleachers watching him play basketball.)