"A 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 stabilise a bubble. A common
misconception is that soap increases the water's surface tension.
Actually soap does the exact 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 causes the surface
tension to increase. Thus, 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.
Their 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, and hence by blowing. If a
bubble is left to sink in still air, however, it remains very
nearly 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, and 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.)"
http://en.wikipedia.org/wiki/Soap_bubble
now i wonder if u geninuely need help in finding ur answer, or u
just can't be bothered to google. 
