human
Why is the Earth's surface covered with water?
assistant
The Earth’s surface is covered in water because it is a planet, and planets are made up of rock and metal. Rock and metal are composed of atoms that have electrons around them. When these atoms bond together, they form molecules. Molecules are made up of atoms bonded together. Atoms can only bond to other atoms if there is space for their electrons. If you look at an atom, you will see that its electron cloud extends far away from the nucleus. This means that atoms can only bond to other atoms if there is room for their electrons.