The levels of nitrogen and _______ in the atmosphere remain fairly constant, but the levels of the other gases may vary. _______, such as carbon dioxide and water vapor, are known to trap heat in Earth's atmosphere. Particles of solids and liquids floating in the atmosphere are known as _______. The upper boundary of the troposphere is called the _______. Ozone gas heats the stratosphere by absorbing _______ rays from the Sun. The International Space Station (ISS) orbits Earth in the _______. In the atmosphere, the _______ has no upper boundary and just fades into space. When you place your hand near a candle flame, your hand will get warm due to _______. If you place one end of a metal rod in a fire, the other end will eventually get hot due to _______. Boiling water on the stove is an example of the transfer of thermal energy via _______.