How to Ruin a Joke

My mom's favorite joke, played on some radio station. ORIGINAL JOKE: Johnny- Mother, can I have another piece of cake? Mother- Yes, you may have more cake, but if you do, you'll explode! Well, Johnny ate another piece of cake and sure enough, he exploded. RIDDLE: What sits next to mother, eats cake and explodes? Give up? JOHNNY! FORGET PUNCHLINE: You- Johnny asks mother if he can have another piece of cake. Mother says yes, but if you do, you'll explode.So then Johnny er... Johnny um Johnnyyyyyyyyyyyyyyyy erm listens to mother?