Soap-water surface tension. It is a kind of gravitational force that pulls water molecules into the smallest possible size. It has been observed that the surface area of a spherical object is the smallest of all the objects it can hold. The area of a sphere made of that amount of material is less than the area of the outer surface of a pyramid , cube , triangle or irregular angle. If you blow on the pipe, the soapy water swells. In this case, as soon as it is released from the pipe, it becomes round due to the effect of surface tension. The air inside the bubble pushes outwards and the force of surface tension holds it evenly. If the air pressure is high, the bubble will burst , and if it is low, it will shrink. The same thing happens with bubble gum. However, instead of surface tension, the elastic strength of the gum rubber works. Elastic forces, such as surface tension, always try to give rubber the smallest size. When the bubble gum is inflated in the air of the mouth, the elastic force of the rubber pushes it inwards. The balance of these two pressures keeps him round like a balloon.