Ahhh Bathwater...The Nectar of the Gods

Why do kids drink bath water? It's such a mystery. It's not JUST the bathwater, it has to be bathwater that's full of baby dirt, soap, shampoo, bubbles, whatever, as long as it's filthy bathwater. I give Cayden about 6 dixie cups when he gets in the tub and I turn on the faucet so that just a small stream comes out so he can fill his cups. Which he does. But then the cups just sit there while he slurps up the gross bathwater like hippo in dirty river. I don't get it. It must be something programmed in them to give them their immunities because so far I've read no obituaries of people dying of bathwater poisoning.