i believe the answer would be gravity. the force of gravity is what makes a liquid substance able to fill the shape of it's container etc. the gravity pulls the water down, which makes it spread until it can't anymore.
gravitational force helps the water to hold onto earth as the gravitational pull holds the water on earth for example if we pour water it goes straight upwards not up in the air ..and this proves that earths gravitational pull helps water stay onto earth
