What Makes Up Bubbles?
Bubbles are generally made of soapy water that has been formed into a thin film. The film traps air in the center, causing the bubble to retain its spherical shape until it pops. The addition of soap to the water is important. The reason why bubbles only really hold their shape when made with soapy water is that the soap stabilizes the surface of the bubble. The soap decreases the bubble's surface tension, which allows it to stretch and hold its shape.
How Bubbles Form
When the soapy solution is stretched across a surface (the end of a bubble wand, for example), it forms a thin, filmy sheet with fairly low surface tension. As air fills the sheet, it takes on a spherical shape. This is because, as the solution stretches and the surface concentration of the soap drops, the surface tension rises. To compensate for this increase in surface tension, the bubble forms into a shape that puts the least amount of stress on the surface layer. For any given volume, spheres have the lowest possible surface area. This means that the surface layer has to stretch the least when forming into a sphere.
Why Do Bubbles Pop?
The addition of soap to the outer layer of the bubble reduces evaporation that plagues bubbles that are formed from water alone (such as bubbles blown in a drink with a straw). However, even soap bubbles burst eventually. This is can happen for a couple different reasons. The first is that soap bubbles are automatically drawn to other objects and bubbles. Generally, when they touch these objects, the bubbles pop. Bubbles also reach their maximum capacity rather quickly, particularly if they are large. This is because, as the soap in the solution spreads out, it's naturally attracted to the weakest points in the bubble. Though the soap stabilizes these weak points, it also tends to prevent them from stretching any further. When pushed beyond their limits, these areas burst.