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: "How can I understand density?"

Response: "By hearing a recording"

A: no

In actual practice, however, we act too often as if we only cared for economic values. If we are to live up to our educational profession, we must look our aim in the face and honestly practise what we believe. While training of character and conduct is the accepted aim for education in general, to make this useful and practical each teacher must fix her attention on how this ultimate aim affects her own special part of the whole work. By watching the free child she will discover how best she can help him: he knows his own business, and when unfettered by advice or command shows plainly that he is chiefly concerned with _gaining experience_. He finds himself in what is to him a new and complex world of people and things; actual experience is the foundation for a complete living, and the stronger the foundation the better the result of later building. _The first vital principle then is that the teacher of young children must provide life in miniature; that is, she must provide abundant raw material and opportunities for experience_.

Question: "The young child and the teacher both highly value what?"

Response: "Freedom of speech"

A: no

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: "Were the key players in the 9/11 conspiracy who executed the plane operations established al Qaeda members?"

Response: "Yes"

A:
yes