The mystery of how the earth remains warm enough to sustain life is one that puzzled scientists for hundreds of years. Then during the nineteenth century, a French scientist developed a theory about how the planet was heated, and the mystery was solved. Jean-Baptiste-Joseph Fourier was curious about the sun?specifically, how it was possible that the sun?s rays hit the oceans and land and did not just bounce back into space. Fourier knew the earth must somehow have the ability to retain the sun?s heat. The very existence of living things was proof of that. So he concluded that only some of the sun?s rays escaped back into space, while some were trapped and held by the earth?s atmosphere. He compared this function to a giant glass vessel, which let in the sun?s light and then trapped and retained its warmth inside.