A soap bubble is a very thin film of soap water that forms a sphere with an iridescent surface. Soap bubbles usually last for only a few moments before bursting, either on their own or on contact with another object. They are often used for children's enjoyment, but their usage in artistic performances shows that they can also be fascinating for adults.

Soap bubbles can help solving complex mathematical problems of space, as they will always find the smallest surface area between points or edges, for example. A soap bubble can exist because the surface layer of a liquid (usually water) has a certain surface tension, which causes the layer to behave somewhat like an elastic sheet. However, a bubble made with a pure liquid alone is not stable and a dissolved surfactant such as soap is needed to stabilize a bubble.

A common misconception is that soap increases the water's surface tension, soap actually does the opposite, decreasing it to approximately one third the surface tension of pure water. Soap does not strengthen bubbles, it stabilizes them, via an action known as the Marangoni effect. As the soap film stretches, the surface concentration of soap decreases, which in turn causes the surface tension to increase.

So soap selectively strengthens the weakest parts of the bubble and tends to prevent them from stretching further. In addition, the soap reduces evaporation so the bubbles last longer, although this effect is relatively small. The spherical shape is also caused by surface tension.

The tension causes the bubble to form a sphere, as a sphere has the smallest possible surface area for a given volume. This shape can be visibly distorted by air currents. However, if a bubble is left to sink in still air, it remains rather spherical, more so, for example, than the typical cartoon depiction of a raindrop.

When a sinking body has reached its terminal velocity, the drag force acting on it is equal to its weight. Since a bubble's weight is much smaller in relation to its size than a raindrop's, its shape is distorted much less. (The surface tension opposing the distortion is similar in the two cases: The soap reduces the water's surface tension to approximately one third, but it is effectively doubled since the film has an inner and an outer surface.)

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