why doesn't the ocean spill into the universe ?
The answer is gravity. gravity is an attractive force and if it is strong enough, not only will water stay but gases like nitrogen and oxygen can stay. Mars is lighter than earth, and so it has less gravity. Mars has a very thin atmosphere because its gravity cannot keep the gases from escaping. The moon is even lighter than Mars, and it has no atmosphere either.
If the earth is flat, then how come the moon, the sun and all the other planets are round? why is the earth the only flat object in the solar system? if the earth is flat, then how come a picture of the earth from outer space shows the earth to be round?