<human>: Why is the Earth's surface covered with water?
<bot>: The Earth’s surface is covered with water because the planet is a sphere. As it rotates, the Earth’s axis of rotation causes different parts of the planet to receive more or less sunlight and heat from the sun. This causes some areas to be warmer than others, which in turn causes evaporation of water from the oceans. This water vapor rises into the atmosphere, condenses into clouds, and eventually falls back down as rain or snow.