Rocky is still playing with the tape measure. Now he’s really having a ball. He pulls the tape out as far as he can, which is about a meter, then releases both the tape and the tape measure at the same time. The resulting recoil and snap-back makes the tape measure spin wildly in front of him.

“♩♪♫♪!!!” he says. I’m pretty sure that was a squeal of glee.

“Look. Look,” I say. “Rocky. Rocky! Yo!”

He finally stops playing with the unintentional toy.

I pull some tape out on my tape measure, then point to the markings. “Look! Here! See these?”

He pulls his own out to approximately the same distance. I can see the markings on his are still there—they didn’t get baked off in the blistering Eridian heat or anything. What is the problem?

I point at the 1-centimeter line. “Look. One centimeter. This line. Here.” I tap the line repeatedly.

He holds the tape out with two hands and taps it with a third. He matches my tempo, but he’s nowhere near the 1-centimeter mark.

“Here!” I tap the mark harder. “Are you blind?!”

I pause.

“Wait. Are you blind?”

Rocky taps the tape some more.

I’ve always assumed he had eyes somewhere and I didn’t recognize them. But what if he doesn’t have eyes at all?

The airlock of the Blip-A was dark, and Rocky didn’t have any problem with it. So I figured he saw in frequencies of light I can’t see. But the tape measure has white tape with black markings on it. Any vision in any spectrum should be able to discern black on white. Black is the absence of light and white is all frequencies equally reflected.

Wait—this doesn’t make sense. He knows what I’m doing. He mimics my gestures. If he doesn’t have vision, how can he read my clock? How can he read his own clock?

Hmm… his clock has thick numbers. Like an eighth of an inch. And, thinking back, he actually did have some trouble with my clock. He needed me to tape it to the divider wall. When it floated an inch away he got upset. Just being close to the divider wasn’t enough. The clock had to be touching it.

“Sound?” I say. “Do you ‘see’ with sound?”

It would make sense. Humans use electromagnetic waves to understand our three-dimensional environment. So why couldn’t a different species use sound waves? Same principle—and we even have it on Earth. Bats and dolphins use echolocation to “see” with sound. Maybe Eridians have that ability, but on steroids. Unlike bats and dolphins, Eridians have passive sonar. They use ambient sound waves to resolve their environment instead of making a specific noise to track prey.

Just a theory. But it fits the data.