Of all the possible shapes in the world, the familiar "double bubble" is the most efficient at enclosing two equal volumes, say two mathematicians from UC Davis and Real Software in Santa Cruz. "There are infinitely many possible shapes for enclosing volumes -- cubes, inner tubes, cell walls, gas tanks," says UC Davis mathematics professor Joel Hass. "As it turns out, nature's soap bubbles are the best." Hass and colleague Roger Schlafly, president of Real Software, proved that two spherical bubbles optimally attached to each other require the least surface area necessary to enclose two equal volumes. The double bubble is familiar to children who have played with bubbles. It can be made by joining two bubbles so they share a flat wall separating the two spheres. The findings may lead to practical applications, especially where efficient containment is important, Hass says. Hass presented the results at a special session on soap bubble science at a mathematics meeting earlier this month. Color images are available in slides and electronic formats.