Baking soda and vinegar sounds more like alchemy - he's listening in an almost amiably grumpy way right up until the other dragon kid starts talking trash about the people who taught him and he tenses up so much he drops the platter and doesn't even notice. He's got two magic teachers - three, now, and he has varying levels of intense respect for all of them. He'd be pretty steamed if Sephiroth were only talking about Elidibus, who has never put himself between Popp and an attacking demon lord and isn't even dead.

One teacher in particular is a huge hot button here, because Popp has modeled half of the things that he actually likes about himself after him.

"Hey! You can say what you like about me, but you don't have the right to insult Master Avan! He knew everything about how magic works, and even more about the human heart! I'd be useless if he'd never taken me in!"

This isn't a fight, and without a fight to put all of those emotions into, they've got to come out one way or another. His vision blurs - oh, those are tears, great, he haaaates it when this happens!