Why was Ueno-san trying to have Tanaka to drink her urine?
Pretty much every episode is about Ueno's scheme to make Tanaka do some silly fetish-esque activity in the hope or either A) make Tanaka aware of her or B) have an opportunity to say some sort of comeback about how much of a pervert he is. It always fails because she either gets hyper-aware of the situation itself while he's always the straight man or she just can't spit it out.