Soap bubbles are always spherical because a sphere provides the smallest surface area for a given volume of air.

The spherical shape of a soap bubble is a result of the physical principle of surface tension, which acts like a stretched elastic skin. This tension is caused by the cohesive forces between liquid molecules, which pull inward to minimize the surface area. Mathematically, the sphere is the unique geometric shape that possesses the smallest surface area for any fixed volume. This minimizes the energy state of the bubble, making it more stable.Soap films are typically composed of a thin layer of water sandwiched between two layers of surfactant molecules. These surfactants, such as sodium lauryl sulfate, reduce the surface tension of pure water just enough to allow the bubble to stretch without popping. When a bubble is blown through a square or triangular wand, it may briefly take an irregular shape. However, as soon as it releases into the air, the internal pressure and external surface tension force it into a sphere within milliseconds.This phenomenon was famously studied by the Belgian physicist Joseph Plateau in the 19th century. In 1873, he published his observations on 'Plateau's Laws,' which describe the structure of soap films in foams. His research demonstrated that the surfaces of these films are always smooth and that their configurations always seek to minimize the total area. Modern researchers at institutions like MIT continue to study these dynamics to improve fluid mechanics and material science.