Attack of the Friendly Donuts (S&BSW)

Premise
Ripple is having trouble understanding what a "donut" is. The Reeftown Rangers fly into the book The Friendly Donut.

Transcript
Bubbles: I'm so glad you're here. Welcome to Reeftown, where all of our saltwater friends live.

...

Bubbles and Ripple were playing finball. Bubbles had brought a donut from her caregiver's house.

Ripple: What's that circle shaped thing?

Bubbles: A donut.

Ripple: I don't get it. How am I supposed to understand what a donut is if I don't get it?