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.

Question: "What can density explain?"

OPTIONS:
- no
- yes

Answer: "Conductivity"

Response: no

According to the radar reconstruction, American 77 reemerged as a primary target on Indianapolis Center radar scopes at 9:05, east of its last known position. The target remained in Indianapolis Center's airspace for another six minutes, then crossed into the western portion of Washington Center's airspace at 9:10. As Indianapolis Center continued searching for the aircraft, two managers and the controller responsible for American 77 looked to the west and southwest along the flight's projected path, not east-where the aircraft was now heading. Managers did not instruct other controllers at Indianapolis Center to turn on their primary radar coverage to join in the search for American 77. In sum, Indianapolis Center never saw Flight 77 turn around. By the time it reappeared in primary radar coverage, controllers had either stopped looking for the aircraft because they thought it had crashed or were looking toward the west. Although the Command Center learned Flight 77 was missing, neither it nor FAA headquarters issued an all points bulletin to surrounding centers to search for primary radar targets. American 77 traveled undetected for 36 minutes on a course heading due east for Washington, D.C. By 9:25, FAA's Herndon Command Center and FAA headquarters knew two aircraft had crashed into the World Trade Center. They knew American 77 was lost. At least some FAA officials in Boston Center and the New England Region knew that a hijacker on board American 11 had said "we have some planes." Concerns over the safety of other aircraft began to mount. A manager at the Herndon Command Center asked FAA headquarters if they wanted to order a "nationwide ground stop." While this was being discussed by executives at FAA headquarters, the Command Center ordered one at 9:25. The Command Center kept looking for American 77. At 9:21, it advised the Dulles terminal control facility, and Dulles urged its controllers to look for primary targets. At 9:32, they found one. Several of the Dulles controllers "observed a primary radar target tracking eastbound at a high rate of speed" and notified Reagan National Airport. FAA personnel at both Reagan National and Dulles airports notified the Secret Service.

Question: "What target crossed into Washington Center's airspace at 9:10?"

Response: "American Flight 77"
OPTIONS:
- no
- yes

A: yes

Problem: Although Bin Laden, Atef, and KSM initially contemplated using established al Qaeda members to execute the planes operation, the late 1999 arrival in Kandahar of four aspiring jihadists from Germany suddenly presented a more attractive alternative. The Hamburg group shared the anti-U.S. fervor of the other candidates for the operation, but added the enormous advantages of fluency in English and familiarity with life in the West, based on years that each member of the group had spent living in Germany. Not surprisingly, Mohamed Atta, Ramzi Binalshibh, Marwan al Shehhi, and Ziad Jarrah would all become key players in the 9/11 conspiracy. Mohamed Atta was born on September 1, 1968, in Kafr el Sheikh, Egypt, to a middle-class family headed by his father, an attorney. After graduating from Cairo University with a degree in architectural engineering in 1990, Atta worked as an urban planner in Cairo for a couple of years. In the fall of 1991, he asked a German family he had met in Cairo to help him continue his education in Germany. They suggested he come to Hamburg and invited him to live with them there, at least initially. After completing a course in German, Atta traveled to Germany for the first time in July 1992. He resided briefly in Stuttgart and then, in the fall of 1992, moved to Hamburg to live with his host family. After enrolling at the University of Hamburg, he promptly transferred into the city engineering and planning course at the Technical University of Hamburg-Harburg, where he would remain registered as a student until the fall of 1999. He appears to have applied himself fairly seriously to his studies (at least in comparison to his jihadist friends) and actually received his degree shortly before traveling to Afghanistan. In school, Atta came across as very intelligent and reasonably pleasant, with an excellent command of the German language. When Atta arrived in Germany, he appeared religious, but not fanatically so. This would change, especially as his tendency to assert leadership became increasingly pronounced. According to Binalshibh, as early as 1995 Atta sought to organize a Muslim student association in Hamburg. In the fall of 1997, he joined a working group at the Quds mosque in Hamburg, a group designed to bridge the gap between Muslims and Christians. Atta proved a poor bridge, however, because of his abrasive and increasingly dogmatic personality. But among those who shared his beliefs, Atta stood out as a decisionmaker.

Question: "What group from Germany became key players in the 9/11 conspiracy"

Answer: "Mohamed Atta, Ramzi Binalshibh, Marwan al Shehhi, and Ziad Jarrah"

OPTIONS:
- no
- yes
Answer: yes

Charlie Mershone had no difficulty in securing his release when Parker came on duty at six o'clock. He called up a cab and went at once to his rooms at the Bruxtelle; and Fogerty followed him. While he discarded his dress-coat, took a bath and donned his walking suit Mershone was in a brown study. Hours ago Louise had been safely landed at the East Orange house and placed in the care of old Madame Cerise, who would guard her like an ogre. There was no immediate need of his hastening after her, and his arrest and the discovery of half his plot had seriously disturbed him. This young man was no novice in intrigue, nor even in crime. Arguing from his own stand-point he realized that the friends of Louise were by this time using every endeavor to locate her. They would not succeed in this, he was positive. His plot had been so audacious and all clews so cleverly destroyed or covered up that the most skillful detective, knowing he had abducted the girl; would be completely baffled in an attempt to find her.

Question: "Were Louise and Charlie in the same place?"

OPTIONS:
- no
- yes

Answer: "They are both at a hotel"

Response:
no