Bubbles on the surface of a liquid are held together by surface tension. Let's look at water... Because water molecules are polar (each end has a different electrical charge), they are attracted to one another. This attraction of water molecules to each other is the source of surface tension. Under the surface, a water molecule share its attraction with molecules on all sides of it. Water molecules on the surface do not have other molecules on all sides of them, so they share extra strong attractions with their neighbors. This occurs whenever water is in contact with air and is called surface tension. Surface tension is what makes bubbles in water round. The surface tension between water molecules pulls them into the tightest package possible, a sphere. As the surface tension pulls the molecules together, the air inside pushes them apart. Eventually, the pulling and pushing reaches equilibrium, and the bubble stays the same size. An exception to the rule is when bubbles rise through water. The pressure on a bubble at the bottom of a container of water is much greater (the weight of the water pushes down on the bubble) than the pressure on the bubble at the surface. Therefore, bubbles at the bottom are smaller than bubbles at the top. So, as a bubble rises through a column of water, it will become larger as it rises. You can probably think of ways to experiment with this at home! Bubbles in a glass container will adhere to the sides of the glass because of the adhesive force between water and glass.