Soap bubbles on a pentagonal prism frame. Bubble films always attempt to occupy the minimum surface area when stretched between two lines, and they meet other films at straight lines. In this frame, a bubble was blown into the centre of the frame, created the pentagonal prism bubble. The internal pressure of this bubble attempts to force it to become spherical, but tension from the frame bubbles constrains it. The balance of forces is responsible for its geometry: a pentagonal prism with curved sides. For an image of the minimum surface area bubble film in this prism, see A350/172.