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Will was riding back to school from a field trip to a museum.

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"The Tyrannosaurus was amazing!" said Bobby, Will's best friend.

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"It was scary!" Will said. He looked at a photo he'd taken of the dinosaur skeleton with his phone and then glanced at their homework.

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"Do you know the answer to number five? Why did dinosaurs disappear?" he asked Bobby.

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"Let's answer these questions before we forget everything . . . or before I fall asleep! Buses make me tired."

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Will tried to focus on the worksheet, but soon his eyes were closed and his head was bobbing up and down with every bump in the road.

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When Will opened his eyes, he wasn't on the bus anymore.

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He was in an aircraft, flying slowly over a strange landscape. QT, his phone, spoke.

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"Scientists think the world looked like this in the late Triassic, the period from about 250 to 200 million years ago."

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"Huh? When?" Will said.

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"This airship is carrying us through time. Scientists talk about Earth's history using a special scale called geologic time.

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See that display? It tells us what period of time we're seeing.

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Experts think dinosaurs, like those over there, appeared during the Triassic.

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But let's travel forward into the next period, when dinosaurs thrived."

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QT shined a light at the controls, taking over.

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The display changed as the scenery outside shimmered and transformed.

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"So is this the Jurassic?" Will asked.

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"Yes. It lasted until about 150 million years ago.

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Some dinosaurs became gigantic, as you can see.

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Let's skip forward into the next period, the Cretaceous.

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At the end of the Cretaceous, 70 percent of the species on Earth died, including most dinosaurs."

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Will nodded. "We learned that at the museum. But what made them disappear?"

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"Well, scientists have some theories, but they can't prove them.

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One clue is an unusual layer of rock found all over the world.

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It's about 65 million years old and contains a lot of iridium, an element that's usually rare on Earth's surface.

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It's normally located deep inside the planet or in space, in some asteroids.

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This layer of rock led to two popular theories about what caused the extinction."

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QT announced, "Here we are—the end of the Cretaceous, 65 million years ago."

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The sky was dark, and they were hovering over a field of volcanoes. "There's so much lava!" Will said.

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"We are over India," QT said.

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"Some scientists believe these volcanoes contributed to the extinction, because they could have brought iridium to Earth's surface in lava.

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The volcanoes might have released enough ash and gases—

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which then blew around the whole planet—to create a greenhouse effect, trapping heat in the atmosphere.

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A warmer climate would have made it hard for many species to survive."

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"Wow," Will whispered, watching a dinosaur flee from the glowing-hot rock.

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"But there is another possibility. Let me transport us to another part of the planet," QT said.

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The ocean appeared on the phone's screen, and the world went pitch-black.

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When it became light again, their aircraft was hovering high over a body of water.

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QT continued, "Some scientists think a huge asteroid, about ten kilometers across, hit Earth near Mexico 65 million years ago."

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A giant rock flew through the air. Will yelled, "Is that the asteroid?"

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"Yes. Scientists are pretty sure it hit Earth, but they can only guess what happened next."

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The aircraft shook as the asteroid crashed into Earth.

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QT went on, "The asteroid might have created a huge fireball and then tsunamis.

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As hot pieces of the asteroid fell back to Earth, they could have started wildfires worldwide.

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So at first the temperature of the world would have increased."

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The aircraft flew closer to the ground, along the shore.

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Will stared at the destruction caused by the asteroid. "How long did this take?"

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"No one knows exactly—maybe weeks or months.

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After the fires dust from the explosion might have blocked light from reaching Earth's surface.

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In the following years, plants, which need light to make food, would have died.

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Then plant-eating dinosaurs would have starved and died.

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And then the meat-eating dinosaurs would have starved and died too."

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"How do we know that?" Will asked.

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"We can guess when species lived based on their fossils and what layer of rock we find those fossils in.

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The species that went extinct don't appear in rocks formed after 65 million years ago."

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Will thought about everything QT had said.

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"But scientists aren't sure whether the volcanoes or the asteroid caused the dinosaur extinction?"

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"It's possible both contributed. There are other factors and theories too," QT replied.

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"For instance, some people think mammals ate so many dinosaur eggs that dinosaurs couldn't survive.

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And we're not sure whether dinosaurs died suddenly.

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They could have been dying off gradually before the volcanoes or asteroid became a problem.”

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QT paused, and the scenery outside began to transform.

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"We've jumped forward just a few years. See all those animals?

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Many mammals, frogs, crocodiles, and even some species of dinosaurs survived the extinction. But we don't know why."

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Will remembered the ferocious-looking Tyrannosaurus at the museum and shuddered.

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"Did some dinosaurs survive into modern times?"

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"Well, scientists are pretty sure birds evolved from dinosaurs, so technically they are dinosaurs.

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And modern crocodiles aren't dinosaurs, but they are very similar to the crocodiles that were alive 65 million years ago," QT said.

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Suddenly something rose out of the water. Its jaws snapped shut on the tail of the aircraft!

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"No!" Will shut his eyes, expecting the crocodile to pull them underwater.

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The aircraft shook and then was still. Will peeked and realized that the bus had jolted to a stop in front of the school.

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"Wake up!" Bobby said. "I hope you had a nice nap!"

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"I had the strangest dream about dinosaurs. I don't think I'll ever forget the stuff I learned today!" Will said.