There are three main reasons why water covers so much of our planet. First, millions of years ago as the Earth was forming, many active volcanoes released water vapor into the atmosphere in the form of steam. Second, the Earth's gravity did not allow that water vapor to escape into space. It collected, along with other gases, to form clouds. Third, as the Earth cooled, the moisture in those clouds condensed (turned from vapor to water), falling to Earth as rain. The rain filled the low areas in the Earth's crust, and the cooler temperatures allowed much of this water to remain in liquid form. Over time, enough water accumulated to create a great ocean. If all of these factors had not been at work over 200,000,000 years ago, our Earth might look much like the Moon does today—dry, barren...