Don't Drink the Bathwater

My son likes to drink bathwater- I'm not sure why but he does. We tell him not to because it will make him sick. Well, he drank the bathwater last night and today his belly hurts. I told him it was because he drank his bathwater but he doesn't believe me- I guess in his almost three year old mind he takes a bath to clean himself so the water must be clean too, right? Oh to be a child again...