I get a large plastic syringe from the supplies. It can pull a maximum of 100 cc of volume.

I attach the syringe to the sippy and unclip the straw. I draw out 100 cc of water, then squirt it into my “wastewater box.” I repeat this a few more times. The last syringe is only about a quarter full when I empty the bag.

Result: 325 ccs of water, which weighs 325 grams! Therefore Rocky’s ball also weighs 325 grams.

I return to the tunnel to tell Rocky all about how smart I am.

He balls a fist at me as I enter. “You left! Bad!”

“I measured the mass! I made a very smart experiment.”

He holds up a string with beads on it. “Twenty-six.”

The beaded string is just like the ones he sent me back when we talked about our atmospheres—

“Oh,” I say. It’s an atom. That’s how he talks about atoms. I count the beads. There are twenty-six in all.

He’s talking about element 26—one of the most common elements on Earth. “Iron,” I say. I point at the necklace. “Iron.”

He points at the necklace and says, “♫♩♪♫♫.” I record the word in my dictionary.

“Iron,” he says again, pointing at the necklace.

“Iron.”

He points to the ball in my hand. “Iron.”

It takes a second to sink in. Then I slap my forehead.

“You are bad.”

It was a fun experiment, but a total waste of time. Rocky was giving me all the information I needed. Or trying to, at least. I know how dense iron is, and I know how to calculate the volume of a sphere. Getting to mass from there is just a little arithmetic.

I pull a pair of calipers out of the toolkit I keep in the tunnel and measure the sphere’s diameter. It’s 4.3 centimeters. From that I work out the volume, multiply by the density of iron, and get a much more precise and accurate mass of 328.25 grams.

“I was only off by one percent,” I grumble.

“You talk to you, question?”

“Yes! I’m talking to me.”

“Humans are unusual.”

“Yes,” I say.

Rocky stretches his legs. “I sleep now.”

“Wow,” I say. This is the first time he’s had to sleep since we met. Good. This will provide me some time for some lab work. But how much time?

“How long do Eridians sleep?”

“I not know.”

“You don’t know? You’re Eridian. How can you not know how long Eridians sleep?”

“Eridians not know how long sleep last. Maybe short time. Maybe long time.”

They sleep unpredictable amounts of time. I guess there’s no rule saying sleep has to evolve as a regular pattern. Does he at least know a range of times it might be?

“Is there a minimum time? A maximum time?”

“Minimum is 12,265 seconds. Maximum is 42,928 seconds.”