That’s why his clock numbers are thick. Because his sonar can’t perceive things that are too thin. My clock was a challenge to him. He can’t “see” the ink, but the hands are solid objects. So he knew about them. But the whole thing is encased in plastic….

I slapped my forehead. “That’s why you needed the clock pressed against the wall. You needed the sound waves bouncing around in it to get to you more easily. And the tape measure I just handed you is useless. You can’t see the ink at all!”

He plays with the tape measure some more.

I hold up a finger. He’s more focused on the tape measure toy, but he absently returns the gesture with one of his spare hands.

I fly back into the ship, through the control room, and into the lab. I grab a screwdriver and head farther down to the dormitory. I detach a storage panel from the floor. Simple aluminum sheet stock. Maybe one-sixteenth of an inch thick, with the edges rounded so we don’t cut ourselves. Strong, durable, and light. Perfect for space travel. I fly back to the tunnel.

Rocky has wrapped one end of the tape around one of his tunnel’s grab-handles and tied it in a somewhat crude knot. He hangs on to the dispenser with one hand and uses the other four to climb backward along the bars.

“Hey,” I say. I hold up my hand. “Hey!”

He stops playing with the tape measure for a moment. “♩♪♩?”

I hold up two fingers.

Rocky holds up two fingers.

“Yeah. Okay. We’re in mimic mode again.” I hold up one finger, then switch to two, then back to one, and then finally three.

Rocky repeats the sequence, just as I hoped he would.

Now I put the aluminum panel between my hand and Rocky. Behind the panel, I hold up two fingers, then one, then three, then five.

Rocky holds up two fingers, then one, then all three. He brings in a second hand to hold up two more fingers for a total of five.

“Wow!” I say.

One-sixteenth-inch aluminum will stop pretty much all light. Some absurdly high frequencies can get through, but those frequencies would also pass right through me. So he wouldn’t see my hands. But sound travels through metals just fine.

That’s proof. He’s not using light to perceive what’s going on. It has to be sound. To Rocky, that metal plate is like a glass window. Maybe it muddles the image a little, but not much. Heck, he probably knows what the Hail Mary’s control room looks like. Why not? The hull is just more aluminum.

How did he see me out in space? No air in space. So no sound.