Everyone loves bubbles. We love to blow them, to pop them, to observe them, and even to drink them in our sodas. Bubbles are almost magical, but why are bubbles round?

To understand why bubbles are round, you need to know about surface tension, and to understand surface tension, you need to know a little bit about water. Water is a molecule made up of two atoms of hydrogen and one atom of oxygen (H20). Water molecules each have six bonds that they can share with their neighboring molecules: one in front and behind, one to the left and to the right, and one above and below. Molecules at the water's surface don't have any neighbors above them to share their top bonds, so they give that extra bond to their neighbor beside them. This extra bond makes all the molecules on the top of the water a little more tightly bound together and creates a custard-skin effect as on pudding, and we call this surface tension.

This effect occurs everywhere water meets air, and in a bubble, every part of the bubble has this air-water interface. You can't blow bubbles with just plain water because the surface tension is too strong to allow the formation of a bubble for any length of time. To blow bubbles you must add soap to the water. Soap decreases the surface tension of water to about one-third of what it usually is and creates a condition that is just right for making bubbles.

Now that we know why the water molecules in bubbles are more tightly tied together on the surface, what makes bubbles round? The surface tension of the water in a bubble pulls the molecules of water into the tightest possible group. The tightest possible grouping that any collection of particles can achieve is to pack together into a sphere. Compared to other shapes such as cubes or pyramids, spheres have the smallest amount of surface area. When you blow a bubble it takes on the shape of a sphere. However, if you did not trap some air inside the bubble, it would continue to shrink until it reached a spherical droplet just like a raindrop.

The air trapped inside the bubble pushes outward against the water film and the surface tension creates an inward force. This pushing inward and outward creates a balance, allowing the bubble to remain the same size. To increase the size of a bubble, you can blow more air into it. More air will make more air pressure inside the bubble, and all the water can do is to expand its surface. Of course, as the bubble grows bigger, the water gets thinner because there are only so many water molecules to go around. If you keep blowing, eventually your bubble will pop.

One challenge in bubble blowing is to make a bubble that can last longer than any other bubble blown. One way to increase your ability to do this is to add a substance to the bubble solution that will keep the water in the solution from evaporating. Water evaporates fairly quickly, and if you can stop the evaporation process, your bubble will last longer. Substances that have water-holding properties are referred to as hygroscopic. Some hygroscopic liquids you can add to your bubble solution are glycerin, sugar, and unflavored gelatin.

Now that you understand a little more about bubbles, it is time to go and mix up some bubble solution and try to blow the largest and longest-lasting bubbles you can. You are never too old to enjoy bubbles and never to young to learn why bubbles are round.