why don't the little ones get tired?

For nearly the last week, ever since I became officially unemployed, I've been helping Kirsten baby-sit her two nephews, one 4-year-old and one 2-nearly-3-year-old. Not quite a week in, and already I'm exhausted. This morning I laid on the floor just on the edge of dozing while they screamed and played and jumped on me. Then I think I'm about the age my dad was when I was 3 or 4 and I'm beginning to realize why sometimes my parents just disappeared or told me to. Also, gaining a whole new respect for what my parents actually accomplished in not killing me (or my siblings, I guess, but probably mostly me).
I do have some experience, however. Yesterday we took the kids to the pet store. As I told someone else, it's a lot like keeping track of people on drugs. They're easily distracted, have 20-second mood swings, no short-term memory and if you do it properly you can use suggestion to give them ideas they think were their own.
And then sometimes they just don't make any sense at all.