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-skee.”
    â€œOh!” said Scott. “So it’s in Russian? No wonder we don’t understand.”
    Mark rolled his eyes. “Math is a universal language, dummy. Uh . . . , isn’t that right, Barry?”
    â€œMore or less,” said Barry. “Now shut up—excuse me, be quiet —and I will explain. So you guys get velocity, right? It’s the speed of something in a particular direction. And that little triangle is a delta sign, which means change. So the left half of the equation is change in velocity. Are you with me so far?”
    Scott and Mark nodded.
    â€œSo the other half of the equation is the exhaust velocity—in other words, how fast the exhaust is spewing out of the rocket’s nozzle. You multiply that times theinitial mass of the rocket, which includes the fuel in the rocket, and then divide it by the final mass of the rocket, which is what’s left after the fuel has burned off. Then that number gets multiplied by the natural log function, and it equals the change in velocity—the left part of the equation.”
    A lightbulb flashed in Mark’s brain. The bulb was only about as big as a firefly, but it was definitely switched on. “So the rocket flies, and as it flies, its mass shrinks because part of its mass was fuel that has burned off in the exhaust?” Mark said. “And as the mass gets smaller, there is less fuel available but, at the same time, it takes less fuel to propel it. Is that right?”
    â€œYes!” said Barry.
    â€œBut what’s this natural log business? It sounds like it has something to do with a fireplace,” Scott asked.
    â€œIn this case it’s a constant to describe something that changes continuously over time,” Barry said. “So the mass of the fuel is changing and affecting the velocity not just moment by moment, but also all the moments in between. The natural log is a mathematical way to describe that.”
    Mark sighed. “If you say so.”
    â€œIt’s Tsiolkovsky who said so,” said Egg.
    â€œIn Russian,” Scott reminded them. “But why do you want to know this again?”
    â€œIt tells you most of what you need to know to puta rocket in space,” Egg said. “How much fuel you need, how massive your rocket and your capsule can be, and how fast you can go. It’s pretty cool.”
    By this time lunch was over, the last lemonade drop drunk and cookie crumb devoured. Grandpa Joe had listened for a while but then gone back into the house to work on today’s remodeling project, a new tile floor in the bathroom. After clearing their dishes and washing up, the kids headed back to the workshop.

CHAPTER 20
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    Egg’s idea was that they make an inventory—a list—of all the items in the workshop, then cross-reference it with what they thought they would need. At that point, they could go back to Nando and see what additional supplies he might be able to provide.
    â€œWe also need to inventory our skill set,” Egg said.
    Mark looked at the other kids one by one, then asked, “Does everybody else understand her all the time?”
    Egg rolled her eyes. “All I mean is, we figure out what each of us knows how to do so we know what help we’ll have to ask for. Like, Howard can program a computer. But besides that there’s going to be metal fabrication, and soldering, and wiring, and plumbing . . . and on and on.”
    â€œI can weld,” said Lisa.
    â€œYou can?” Scott said. “Can you teach us?”
    Lisa looked at her toes and nodded.
    â€œThat is so cool!” said Mark. “How does welding work, exactly?”
    Lisa thought for a second. “Basically, you’re using an arc of electricity to melt metal and make glue out of it. Then the glue sticks two pieces of metal together.”
    â€œIs it hard?” Egg asked.
    â€œIt’s hard to do neatly,” said Lisa.