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em_spectrum.txt

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+The electromagnetic (EM) spectrum is the range of all types of EM radiation.
+Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. 
+The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays. You know more about the electromagnetic spectrum than you may think. 
+The image below shows where you might encounter each portion of the EM spectrum in your day-to-day life. 
+Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space. 
+Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies. 
+Night vision goggles pick up the infrared light emitted by our skin and objects with heat.
+In space, infrared light helps us map the dust between stars. 
+Our eyes detect visible light. 
+Fireflies, light bulbs, and stars all emit visible light. 
+Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns. 
+"Hot" objects in space emit UV radiation as well. 
+A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. 
+Hot gases in the Universe also emit X-rays. 
+Doctors use gamma-ray imaging to see inside your body. 
+The biggest gamma-ray generator of all is the Universe. 
+Is a radio wave the same as a gamma ray? 
+Are radio waves completely different physical objects than gamma-rays? 
+They are produced in different processes and are detected in different ways, but they are not fundamentally different. 
+Radio waves, gamma-rays, visible light, and all the other parts of the electromagnetic spectrum are electromagnetic radiation. 
+Electromagnetic radiation can be described in terms of a stream of mass-less particles, called photons, each traveling in a wave-like pattern at the speed of light. 
+Each photon contains a certain amount of energy. 
+The different types of radiation are defined by the amount of energy found in the photons. 
+Radio waves have photons with low energies, microwave photons have a little more energy than radio waves, infrared photons have still more, then visible, ultraviolet, X-rays, and, the most energetic of all, gamma-rays. 
+Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency.
+ Frequency is measured in cycles per second, or Hertz. Wavelength is measured in meters.
+ Energy is measured in electron volts. 
+Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way.

water_cycle.txt

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+The water cycle describes how water moves through Earth's systems.
+Water evaporates from oceans, lakes, and rivers.
+It rises into the atmosphere and cools to form clouds.
+This process is called condensation.
+Eventually, the water falls back to Earth as precipitation, such as rain or snow.
+Some of this water runs off into bodies of water, and some soaks into the ground.
+This groundwater may be taken up by plants or may eventually return to lakes and oceans.
+The cycle then repeats.