warm suds on a hot day.

I use a cup to rinse Alex's hair in the bath. The other night He tried to drink bathwater from the cup. So I took the cup out and started using the toy boat and he tried to drink from the water from it as I poured it on him. Tonight he decided to forgo the middle man and stick his face in the water to drink. Why is it that he won't drink water from his sippy cup but he wants to drink his bath water?