QUES: 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.
Do you have any questions?
MY QUESTION: What are three examples of density given in the article?

QUES: You may be wondering, how can a glacier get so big? Why does it move? These are both good questions. In the winter months, precipitation falls as snow. This solid form of water builds up on the ground as long as the temperatures stay cold enough. As the temperature rises, the snow starts to melt. The frozen water changes state back into a liquid state. Nearer the poles, summer does not last very long. If the summer is long enough and warm enough, all the snow may melt. This is what typically happens now. The earth was a little cooler 12,000 years ago. As a result, during the summer months, that amount of snow did not melt. It may have only been an inch or so of snow that melted. The following winter, snow fell on top of this left-over snow. This next winters snowfall had a head start. Year after year, the snow that did not melt became thicker and thicker. Inch by inch the snow started to build up. Over many years, layer upon layer of snow compacted and turned to ice.
Do you have any questions?
MY QUESTION: How does the snow turn into ice?

QUES: It is not enough to describe some objects by their location. We may also need to know its direction. For example, a wind blows a storm over your school. Where is that storm coming from? Where is it going? How can we describe a direction? The most common way is by using a compass. A compass is a device with a floating needle (Figure 2.1). The needle is a small magnet. The needle aligns itself with the Earth's magnetic field. The compass needle always points to magnetic north. If you have a compass and you find north. You can then know any other direction. See the directions, such as east, south, west, etc., on a compass rose. The Earth has two poles. It has a magnetic north pole. It also has a geographic north pole. These two poles are close to each other but not the exact same place.
Do you have any questions?
MY QUESTION: How does the compass work?

QUES: 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.
Do you have any questions?
MY QUESTION:
Who was standing next to the president when his reaction was that the crash of American 11 must be pilot error?