Scientists have finally figured the reason why bottled beer foams over a tap on the mouth. This has to do with fluid mechanics.

When a compression wave travels down through the glass, it hits the bottom and is reflected as an expansion wave that travels through the beer. The waves keep bouncing back and forth until the compression waves break up the CO2 bubbles in your beer into thousands of tiny microbubbles.

The microbubbles expand violently because of the expansion waves, which then expand into skyrocketing plumes. This results in millions of expanding CO2 bubbles which turn your beer into foam shooting out of the bottle. [Francis WorldInsideOut via Smithsonian]