"Atoms. Elements. Look...." I grabbed up a fistful of the sheets and thumbed through them. "Look ... each message starts with a list of numbers. Then there's a long blank to separate the opening list from the rest of the message. See? Every time, the same length list." "So?" "The periodic table of the elements!" I shouted into his ear. "That's the key!" Rizzo shook his head. "I thought of that two days ago. No soap. In the first place, the list that starts each message isn't always the same. It's the same length, all right, but the numbers change. In the second place, it always begins with 100000. I looked up the atomic weight of hydrogen--it's 1.008 something." That stopped me for a moment. But then something clicked into place in my mind. * * * * * "Why is the hydrogen weight 1.008?" Before Rizzo could answer, I went on, "For two reasons. The system we use arbitrarily rates oxygen as 16-even. Right? All the other weights are calculated from oxygen's. And we also give the average weight of an element, counting all its isotopes. Our weight for hydrogen also includes an adjustment for tiny amounts of deuterium and tritium. Right? Well, suppose _they_ have a system that rates hydrogen as a flat one: 1.00000. Doesn't _that_ make sense?" "You're getting punchy," Rizzo grumbled. "What about the isotopes? How can they expect us to handle decimal points if they don't tell us about them ... mental telepathy? What about...." "Stop arguing and start calculating," I snapped. "Change that list of numbers to agree with our periodic table. Change 1.00000 to 1.008-whatever-it-is and tackle the next few elements. The decimals shouldn't be so hard to figure out." Rizzo grumbled to himself, but started working out the calculations. I stepped over to the dome's microspool library and found an elementary physics text. Within a few minutes, Rizzo had some numbers and I had the periodic table focused on the microspool reading machine. "Nothing," Rizzo said, leaning over my shoulder and looking at the screen. "They don't match at all." "Try another list. They're not all the same." He shrugged and returned to his desk. After a while he called out, "their second number is 3.97123; it works out to 4.003-something." It checked! "Good. That's helium. What about the next one, lithium?" "That's 6.940." "Right!" Rizzo went to work furiously after that. I pushed a chair to the desk and began working up from the end of the list. It all checked out, from hydrogen to a few elements beyond the artificial ones that had been created in the laboratories here on Earth. "That's it," I said. "That's the key. That's our Rosetta Stone ... the periodic table." Rizzo stared at the scribbled numbers and jumble of papers. "I bet I know what the other lists are ... the ones that don't make sense." "Oh?"