Density is also an important physical property of matter. The concept of density combines what we know about an objects mass and volume. Density reflects how closely packed the particles of matter are. When particles are packed together more tightly, matter is more dense. Differences in density of matter explain many phenomena. It explains why helium balloons rise. It explains why currents such as the Gulf Stream flow through the oceans. It explains why some things float in or sink. You can see this in action by pouring vegetable oil into water. You can see a colorful demonstration in this video.
What question would one ask from this paragraph?
Answer: How can I understand density?

When American 11 struck the World Trade Center at 8:46, no one in the White House or traveling with the President knew that it had been hijacked. While that information circulated within the FAA, we found no evidence that the hijacking was reported to any other agency in Washington before 8:46. Most federal agencies learned about the crash in New York from CNN. Within the FAA, the administrator, Jane Garvey, and her acting deputy, Monte Belger, had not been told of a confirmed hijacking before they learned from television that a plane had crashed. Others in the agency were aware of it, as we explained earlier in this chapter. Inside the National Military Command Center, the deputy director of operations and his assistant began notifying senior Pentagon officials of the incident. At about 9:00, the senior NMCC operations officer reached out to the FAA operations center for information. Although the NMCC was advised of the hijacking of American 11, the scrambling of jets was not discussed. In Sarasota, Florida, the presidential motorcade was arriving at the Emma E. Booker Elementary School, where President Bush was to read to a class and talk about education. White House Chief of Staff Andrew Card told us he was standing with the President outside the classroom when Senior Advisor to the President Karl Rove first informed them that a small, twin-engine plane had crashed into the World Trade Center. The President's reaction was that the incident must have been caused by pilot error. At 8:55, before entering the classroom, the President spoke to National Security Advisor Condoleezza Rice, who was at the White House. She recalled first telling the President it was a twin-engine aircraft-and then a commercial aircraft-that had struck the World Trade Center, adding "that's all we know right now, Mr. President." At the White House, Vice President Dick Cheney had just sat down for a meeting when his assistant told him to turn on his television because a plane had struck the NorthTower of the World Trade Center. The Vice President was wondering "How the hell could a plane hit the World Trade Center" when he saw the second aircraft strike the South Tower. Elsewhere in the White House, a series of 9:00 meetings was about to begin. In the absence of information that the crash was anything other than an accident, the White House staff monitored the news as they went ahead with their regular schedules.
What question would one ask from this paragraph?
Answer: What is the NMCC and when did their senior operations officer discuss scrambling jets?

Jack and Mackenzie wanted to do something fun during their day off from school. They knew that the library had story time on Monday, Tuesday, and Wednesday. Their mother told them they could go to the library every Tuesday for their story time. They packed up the car and drove to the library, ready for a fun morning. When they arrived to the story room, there were lots of other children their age, all sitting cross-legged and ready for the story of the day. The teacher told them they would be reading "The Wild Horse". She began to tell the story of Majestic, the wild horse who could not be calmed. His people had tried and tried to break him, but he was wild at heart. Every time they took him to the river to drink, he would run straight into the water and get soaking wet! He would splash and play until he was ready to go back home, where he would sleep for hours, having worn himself out playing.
What question would one ask from this paragraph?
Answer: Majestic was the main character of which story?

Take a look at this train in Figure 1.11. It looks very futuristic. What do you notice about it? Did you notice that the train has no wheels? How can a train have no wheels? It doesn't need wheels. It actually floats, or levitates, just above the track. Magnets enable the train to do this. This is not a normal train. This is a maglev train. The word maglev stands for magnetic levitation. Because it has no wheels, there is no friction. Some magnets hold the train up. Other magnets are used to move the train forward. This train can go very fast. It can reach speeds up to 480 kilometers (300 miles) per hour! Magnets are pretty cool. What exactly is a magnet? How is it able to exert such force?
What question would one ask from this paragraph?
Answer:
How does the train move?