The video is being made. Until it arrives, here is what it will say (117 words, about 58 seconds):

Here's Tallis in the aquarium tunnel. A shark glides overhead, a jellyfish drifts past, a crab scuttles by, and an octopus clings to the glass.

Four animals that look nothing alike, yet the sign says three of them go in one group, and the shark is on its own. Which three, and why?

Tallis guesses it's size, but the octopus is nearly as big. Not legs either: the jellyfish has none, the crab has ten.

So what are the sign-makers looking at? They sort every animal this way, one side or the other.

That's what this topic is about: sorting animals into groups by what their bodies have.

Okay, so are you ready to start sorting animals?

It ends with a ready line. Then pick a reply below — any reply counts.