A bubble is a thin film of soapy water. Most of the bubbles that you see are filled with air, but you can make a bubble using other gasses, such as carbon dioxide. The film that makes the bubble has three layers. A thin layer of water is sandwiched between two layers of soap molecules. Each soap molecule is oriented so that its polar (hydrophilic) head faces the water, while its hydrophobic hydrocarbon tail extends away from the water layer. No matter what shape a bubble has initially, it will try to become a sphere. The sphere is the shape that minimizes the surface area of the structure, which makes it the shape that requires the least energy to achieve.
When bubbles stack, do they remain spheres? No. When two bubbles meet, they will merge walls to minimize their surface area. If bubbles that are the same size meet, then the wall that separates them will be flat. If bubbles that are different sizes meet, then the smaller bubble will bulge into the large bubble. Bubbles meet to form walls at an angle of 120 degrees. If enough bubbles meet, the cells will form hexagons. You can see observe this structure by making prints of bubbles or by blowing bubbles between two clear plates.
