Jon frantically tried to mouth out that he couldn't speak.
Becky rolled her eyes and said "Okay Jon you forced my hand. I wish that Jon would eat this slice of bread."
Against his will, Jon took the slice and obediently ate it. To his surprise however it didn't taste like anything at all. It had been part of him, so to him it tasted like nothing at all. (similar to being unable to smell your own smell)
Becky's face contorted with disgust. "Ew Jon, that's just nasty. You're eating yourself. Like you're a cannibal or something"
Jon glared at her. He was really starting to get pissed off at her. If only he could get the stone, then he would... what? what would he do? He was mute. he couldn't make a wish. He'd have to get help. Karyn maybe? But he couldn't call her and he certainly leave Becky alone with the stone. What if she made a wish and Jon couldn't hear it? No... the best thing to do is play along with Becky for the moment and hope that Karyn decides to comes over.