Soap bubbles on a triangular 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 second bubble was blown into the part at bottom. The presence of this bubble has distorted its shape, which was previously tetrahedral (as at the top triangle). The internal pressure of the new bubble tries to force it into a spherical shape, but the frame bubbles constrain it in a tetrahedral form. The resulting geometry is a combination between these two extremes: a tetrahedral bubble with curved sides.