Objects floating on the surface of fluid or immersed in fluid are subjected to the upward resultant force of hydrostatic pressure from all directions. Its size is equal to the gravity of the fluid displaced by the object. For example, if the gravity of a stone is greater than that of the same volume of water, it will sink to the bottom of the water. The gravity of wood or hull is equal to the weight of water displaced by the submerged part, so it floats on the water. The weight of the balloon is less than the gravity of the same volume of air, that is, the buoyancy is greater than gravity, so it will rise. This kind of force immersed in water or air and pulled up by water or air is called "buoyancy". For example, lifting a bucket of water from a well is lighter before leaving the water surface than after leaving the water surface, because the bucket is buoyed by the water. Not only water, but also all liquids, such as alcohol, kerosene or mercury, have buoyancy for objects immersed in it.