Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Bacteria change nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb. Answer: bacteria

A: What changes nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb?
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Q: Passage: Light may transfer its energy to matter rather than being reflected or transmitted by matter. This is called absorption . When light is absorbed, the added energy increases the temperature of matter. If you get into a car that has been sitting in the sun all day, the seats and other parts of the car's interior may be almost too hot to touch, especially if they are black or very dark in color. That's because dark colors absorb most of the sunlight that strikes them. Answer: absorption

A: When light transfers its energy to objects, this is known as what?
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Q: Passage: All gases exert pressure; it is one of the fundamental measurable quantities of this phase of matter. Even our atmosphere exerts pressurein this case, the gas is being "held in" by the earth's gravity, rather than the gas being in a container. The pressure of the atmosphere is about 14.7 pounds of force for every square inch of surface area: 14.7 lb/in2. Pressure has a variety of units. The formal, SI-approved unit of pressure is the pascal (Pa), which is defined as 1 N/m2 (one newton of force over an area of one square meter). However, this is usually too small in magnitude to be useful. A common unit of pressure is the atmosphere (atm), which was originally defined as the average atmospheric pressure at sea level. However, "average atmospheric pressure at sea level" is difficult to pinpoint because of atmospheric pressure variations. A more reliable and common unit is millimeters of mercury (mmHg), which is the amount of pressure exerted by a column of mercury exactly 1 mm high. An equivalent unit is the torr, which equals 1 mmHg. (The torr is named after Evangelista Torricelli, a seventeenth-century Italian scientist who invented the mercury barometer. ) With these definitions of pressure, the atmosphere unit is redefined: 1 atm is defined as exactly 760 mmHg, or 760 torr. We thus have the following equivalences: 1 atm=760 mmHg=760 torr. Answer: gravity

A:
The pressurized gas of earth's atmosphere is contained by what force?
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