Your question is a good question. Most
three-dimensional things that are perfectly round
in all directions are called "spheres".
Therefore, bubbles are said to be "spherical."
Most things that are spherical are shaped in this
way because of two reasons (that I know of):
1) the forces pushing on the bubble from both inside and outside are equal in all directions, causing the surface to be completely smooth and uniform with no corners or edges; or
2) the stuff on the inside of the bubble wants to minimize its contact with the stuff on the outside of the bubble, perhaps because of some sort of repulsive forces (like when you hold two positive magnets together).For instance, since water and oil don't mix because they repel one another, if you add oil to water, the oil will form small bubbles that are nearly spherical.
Spherical bubbles minimize the amount of contact between the inside and outside substances because spheres have the smallest surface area of any shape, for a given volume. If you take a sphere (like a bubble) and a cube (shaped like a box) that have the exact same volume (or can hold the same amount inside them), the sphere will have a much smaller
surface area than the box. If the substance
inside the bubble is repelled from the substance
outside the bubble, the substance inside the
bubble will try to group together into a shape
that minimizes its contact with the outside and will in this way form a spherical bubble. The force that pulls the substance inside a bubble together and tries to keep the spherical shape is called "surface tension."
Keep in mind that for both of the two explanations I gave, spherical bubbles will usually only form if the substance is
a fluid (something that flows, like a liquid or gas) and can be bent and formed into a sphere by the forces pushing on it. Raindrops are in a sense a type of bubble because the water is
repelled from the surrounding air and forms a
sphere. But there are a lot of bubbles that
are NOT spherical.
If a bubble is moving through another fluid, it can lose its spherical (round) shape because the forces pushing on the bubble from the outside change as it moves through a gas or liquid. For instance, if you go SCUBA diving you will notice that the air bubbles that you blow out into the water are less dense than the water and repelled from it, so they form spherical bubbles that begin to rise to the surface. When the bubbles rise to the surface, they expand and are pushed against by the water above them that they are moving through. If the bubbles are big enough and rise fast enough, then the force of the water pushing against the top of the bubble causes the larger bubbles to flatten. These bubbles change shape into a bowl, with a rounded top and a flattened bottom. This flattening effect also
happens to large raindrops as they fall from the sky and are flattened by the force of the air
pushing against them from below. The larger
bubbles of carbon dioxide that form and rise in a
soda bottle after opening it probably also flatten
in this way.
The flattening of bubbles only happens to larger raindrops and air bubbles as they move, because in order for the forces pushing on the outside of the bubble to cause it to flatten, these forces have to exceed the surface tension that is trying to pull the drop into a spherical shape in order to minimize its surface area.
The force of the fluid pushing against the outside of the bubble or raindrop is only greater than the surface tension for larger bubbles and raindrops. This is because the larger ones rise (in the case of bubbles) or fall (in the case of raindrops) faster than small ones and so
the force on their outside is greater.