Respiring yeast cells release carbon dioxide which gets trapped and so forms tiny gas bubbles.

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When water velocity is high, the pressure within the liquid drops to below the vapour pressure of water, allowing tiny air bubbles to expand.

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So we opted for a sombre burial in the Greenhouse, under the shade of the tallest of the mangroves, though in truth it meant a slightly watery grave after all, stuff seeping up through the muck to swallow the foot, bubbles of mud detaching and floating among us during our tiny, foot-size ritual observances.

NEWYORKER: Lostronaut