Bubbles occur when a thin film (for example, of soapy water) traps some gas (for example, air). The molecules in the film are attracted to each other, which not only holds the film together, but also makes it shrink to the smallest possible area.
The smallest area enclosing any given volume? A sphere. Therefore, the film will shrink to cover a sphere, and then can’t shrink any further because of the trapped air. Thus, bubbles end up as round.
Asked by: Emily Wilson, Birmingham
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Jon is a Professor of Physics at University College London. He works on the ATLAS experiment at the CERN Large Hadron Collider.