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"I don't get this," Will Jacobs grumbled as he tried to do his fourth-grade science homework.

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His mother stepped into the doorway and asked, "Will, are you done with your homework? It's almost time for bed."

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"No. I'm still trying to answer these questions.

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I just don't understand earthquakes. How can the earth shake?"

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"Why don't you take a little break?" Will's mom suggested. "That sometimes helps."

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After his mom left, Will set an alarm on his phone.

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He had just gotten it for his birthday, and he loved the new gadget.

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Will put his head down on the desk and instantly fell asleep.

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Soon the alarm went off, making the table vibrate.

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Will silenced it, ignoring the message on the phone's screen: "WAKE UP!"

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He went back to sleep, only to be woken up again by another vibration.

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But this time when Will opened his eyes, he wasn't in his room—he was in a dark, long cave.

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And the phone wasn't vibrating—the ground was shaking!

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"Whoa," thought Will. "What's going on?"

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A voice nearby squeaked, "Alert! Earthquake!"

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Will looked around but didn't see anyone. Then he spotted a patch of light on the ground.

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He looked closer and saw that it was his phone. The voice came from the device!

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Will picked up the phone, mumbling to himself, "This is strange. Phones can't talk!"

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"Most phones don't talk, but I am a QT-9000."

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He almost dropped it again when the phone replied!

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"I can talk and do much more. I'm the smartest phone you'll ever have! You can call me QT."

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"Wow!" Will exclaimed. "Can you tell me what's happening? Where am I?"

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"My sensors indicate that we are in a cave.

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And we just experienced a small earthquake," replied QT.

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All of a sudden, the ground started shaking again.

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"And this is a big earthquake," QT said as a whole section of the rock wall moved.

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Will noticed light coming through a crack in the ceiling.

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"I'm getting out of here," he said, stuffing the phone in his pocket. He climbed out to the surface.

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Nearby a road had torn in half. Will pulled his phone out and asked, "How can a huge part of the ground move like that?"

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A map of the world appeared on QT's screen.

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"The surface of the planet, called the crust, is made up of chunks called tectonic plates.

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There are 12 large plates and many smaller ones," QT explained.

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"The plates float on top of the mantle, a soft layer of rock and other materials.

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Underneath the mantle is a layer of liquid metal called the outer core.

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And underneath that is the inner core, which is solid metal.

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It's very hot at the center of the planet, and the heat escaping from there makes the plates move.

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The movement is what causes earthquakes," said QT.

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"Oh . . ." Will thought for a moment and then asked, "If the plates are constantly moving, why aren't earthquakes happening all the time?"

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"Actually they are! Most are just so small that people don't feel them.

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Big earthquakes, like this one, happen when friction stops the edges of the plates from moving for a while.

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When they finally break loose, they release all that stored energy, like a spring." QT paused for a second.

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"My GPS tells me that we are on the San Andreas Fault, in California."

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Will's eyes lit up. "We learned about faults in science class!

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A fault is the crack between two plates in the planet's crust!"

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Will looked at the road again. "That must be a fault over there."

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"Yes, it is. Two plates are sliding past each other here, causing powerful earthquakes.

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Faults also often appear by mountains. That's because when two plates come together, their edges can wrinkle and fold to make mountains."

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Will scratched his head. "What happens when plates move apart? Do huge holes appear in the ground?"

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"No. Right now the places where plates are moving apart are mostly under the ocean.

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Down there magma—that's hot, liquid rock—rises from between the plates.

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It cools when it touches the cold water, so it hardens and becomes part of the crust," QT explained. "Earthquakes can be very destructive.

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If that road over there is destroyed, just imagine what can happen in a city during an earthquake. Actually I'll show you."

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"How? We're nowhere near a city," Will pointed out.

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"Look at my screen and I'll transport us. I told you I have special skills."

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Will gazed down at the screen and saw a street in ruins.

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Then everything got dark. A moment later Will found himself in the city!

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He was standing in a crowded park.

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Will was shocked by the destruction around him.

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"I'd better stay here, away from the buildings.

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My science teacher said that aftershocks always come after a big earthquake, but we don't know when.

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It's dangerous because more parts of the buildings could fall. But where are we?" asked Will.

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"This is San Francisco after a magnitude 6.9 earthquake," QT replied.

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"The earthquake's epicenter was actually about a hundred kilometers away, but you can see how much power it released."

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Will looked confused. "What's an epicenter, and what does magnitude mean?"

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"Earthquakes start deep underground. The spot on the surface of the planet above the earthquake is called its epicenter.

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Magnitude is the strength of an earthquake.

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Each point on the magnitude scale is ten times stronger than the one before, so an 8-point earthquake is ten times stronger than a 7-point.

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Can you imagine damage ten times worse than this?"

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Will shook his head. "No. This is pretty bad. How do they measure magnitude?

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Do they count the buildings that fell or something?"

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A jagged line appeared on QT's screen. "People who study earthquakes are called seismologists.

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They measure earthquakes with machines called seismographs, which monitor the vibrations in the ground.

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Usually the earth doesn't move much, so the waves the seismographs draw are pretty small.

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But during an earthquake, the waves get really big.

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The size of the waves determines the magnitude."

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Suddenly the aftershock came. The ground rumbled and then moved.

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Terrified, Will shut his eyes. When the sound passed, he slowly opened his eyes.

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To his amazement he was back in his room. His phone was vibrating in his hand; the alarm was going off.

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"That was a strange dream . . . ," he muttered, reaching for his pencil.

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"But QT sure is cool! And now these homework questions will be a piece of cake!"