The kids a playing “pirate ship”.

Ezra: “Man the cannon! The enemy is upon us!!”

(later)

Phineas: “Phhhhhhhhh! Nate, you’re on fire!”
Me: “Hm. That doesn’t seem very nice.”
Phineas: “That’s because… WE’RE PIRATES!!!! Aaarrgh!”

Then they both shouted “aargh” at me a lot.