pee in someone's Cheerios

— “Whatever, I just don’t know how to make Elizabeth believe me. Brianna said that if I don’t convince Elizabeth to quit the team, she’s going to make us pay. Elizabeth is brand-new but totally outswam Brianna during tryouts. She’s super jealous!” I explain, resting my chin in my hands. / “I hear you,” Frank says, setting his knitting needles aside. “That little whippersnapper’s a piece of work. Who peed in her Cheerios anyway?”