Will was riding back to school from a field trip to a museum.
"The Tyrannosaurus was amazing!" said Bobby, Will's best friend.
"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.
"Do you know the answer to number five? Why did dinosaurs disappear?" he asked Bobby.
"Let's answer these questions before we forget everything . . . or before I fall asleep! Buses make me tired."
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.
When Will opened his eyes, he wasn't on the bus anymore.
He was in an aircraft, flying slowly over a strange landscape. QT, his phone, spoke.
"Scientists think the world looked like this in the late Triassic, the period from about 250 to 200 million years ago."
"Huh? When?" Will said.
"This airship is carrying us through time. Scientists talk about Earth's history using a special scale called geologic time.
See that display? It tells us what period of time we're seeing.
Experts think dinosaurs, like those over there, appeared during the Triassic.
But let's travel forward into the next period, when dinosaurs thrived."
QT shined a light at the controls, taking over.
The display changed as the scenery outside shimmered and transformed.
"So is this the Jurassic?" Will asked.
"Yes. It lasted until about 150 million years ago.
Some dinosaurs became gigantic, as you can see.
Let's skip forward into the next period, the Cretaceous.
At the end of the Cretaceous, 70 percent of the species on Earth died, including most dinosaurs."
Will nodded. "We learned that at the museum. But what made them disappear?"
"Well, scientists have some theories, but they can't prove them.
One clue is an unusual layer of rock found all over the world.
It's about 65 million years old and contains a lot of iridium, an element that's usually rare on Earth's surface.
It's normally located deep inside the planet or in space, in some asteroids.
This layer of rock led to two popular theories about what caused the extinction."
QT announced, "Here we are—the end of the Cretaceous, 65 million years ago."
The sky was dark, and they were hovering over a field of volcanoes. "There's so much lava!" Will said.
"We are over India," QT said.
"Some scientists believe these volcanoes contributed to the extinction, because they could have brought iridium to Earth's surface in lava.
The volcanoes might have released enough ash and gases—
which then blew around the whole planet—to create a greenhouse effect, trapping heat in the atmosphere.
A warmer climate would have made it hard for many species to survive."
"Wow," Will whispered, watching a dinosaur flee from the glowing-hot rock.
"But there is another possibility. Let me transport us to another part of the planet," QT said.
The ocean appeared on the phone's screen, and the world went pitch-black.
When it became light again, their aircraft was hovering high over a body of water.
QT continued, "Some scientists think a huge asteroid, about ten kilometers across, hit Earth near Mexico 65 million years ago."
A giant rock flew through the air. Will yelled, "Is that the asteroid?"
"Yes. Scientists are pretty sure it hit Earth, but they can only guess what happened next."
The aircraft shook as the asteroid crashed into Earth.
QT went on, "The asteroid might have created a huge fireball and then tsunamis.
As hot pieces of the asteroid fell back to Earth, they could have started wildfires worldwide.
So at first the temperature of the world would have increased."
The aircraft flew closer to the ground, along the shore.
Will stared at the destruction caused by the asteroid. "How long did this take?"
"No one knows exactly—maybe weeks or months.
After the fires dust from the explosion might have blocked light from reaching Earth's surface.
In the following years, plants, which need light to make food, would have died.
Then plant-eating dinosaurs would have starved and died.
And then the meat-eating dinosaurs would have starved and died too."
"How do we know that?" Will asked.
"We can guess when species lived based on their fossils and what layer of rock we find those fossils in.
The species that went extinct don't appear in rocks formed after 65 million years ago."
Will thought about everything QT had said.
"But scientists aren't sure whether the volcanoes or the asteroid caused the dinosaur extinction?"
"It's possible both contributed. There are other factors and theories too," QT replied.
"For instance, some people think mammals ate so many dinosaur eggs that dinosaurs couldn't survive.
And we're not sure whether dinosaurs died suddenly.
They could have been dying off gradually before the volcanoes or asteroid became a problem.”
QT paused, and the scenery outside began to transform.
"We've jumped forward just a few years. See all those animals?
Many mammals, frogs, crocodiles, and even some species of dinosaurs survived the extinction. But we don't know why."
Will remembered the ferocious-looking Tyrannosaurus at the museum and shuddered.
"Did some dinosaurs survive into modern times?"
"Well, scientists are pretty sure birds evolved from dinosaurs, so technically they are dinosaurs.
And modern crocodiles aren't dinosaurs, but they are very similar to the crocodiles that were alive 65 million years ago," QT said.
Suddenly something rose out of the water. Its jaws snapped shut on the tail of the aircraft!
"No!" Will shut his eyes, expecting the crocodile to pull them underwater.
The aircraft shook and then was still. Will peeked and realized that the bus had jolted to a stop in front of the school.
"Wake up!" Bobby said. "I hope you had a nice nap!"
"I had the strangest dream about dinosaurs. I don't think I'll ever forget the stuff I learned today!" Will said.