In "Night of the Living Rez," what does David think the woman has in the Gatorade bottle, and how does he later learn what the liquid really is?

When David is on the bus with the woman, he thinks she has a bottle of apple juice. Later, his mother is watching the news and tells him that a woman used a water gun to spray people in a store with her own urine, and David realizes that this is the woman from the bus--and that the bottle contained urine.