This video excerpt from NOVA explains what astrobiologists consider to be the three basic ingredients of life—water, organic compounds, and energy. Although Earth has water and energy, scientists are trying to learn how organic molecules got to our planet. Some believe that comets could help us understand how the final ingredients necessary for life arrived on Earth. In activity two from the Education Collection that accompanies this video, students observe a number of objects, make a list of life’s characteristics, and develop a working definition of being alive.

This video is available in both English and Spanish audio, along with corresponding closed captions.