Heads you win, tails I lose

Isaac in 100 words or less: Asks for water, but does not want the water in his glass. The water in his class must be poured into my empty glass before judged acceptable. Then proceeds, repeatedly, increasingly aggressively, to offer me his water, holding the cup to my lips and refusing to be fooled by my pretend sips. Finally, to placate him, I drink some. Whereupon he screeches, “My water! Mine!” Whereupon I tickle him.