He's ready to launch fire, but he actually decides to listen to a little bit of what Gustav is saying, out of some tiny, nagging tidbit of curiosity he's feeling.

But the curiosity quickly goes away as soon as Scout realizes that this lecture is totally boring and his job is to spice it up a little.

"ACH -- ptoo!"

Does it land on the professor? Scout can't see, because he's currently laughing with his eyes closed.