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Some bacteria live in the mud at the bottom of the sea. As long as they are in the mud, all is well. Suppose that the mud gets stirred up, perhaps by a crustacean walking by. Now things are not so rosy. The bacteria cannot survive for long in the water, so it is imperative that they swim back down to the mud as soon as possible. The problem is that knowing which direction is down is not so easy. The mass density of the bacteria is almost identical to that of water, so the buoyant force prevents them from "feeling" the downward pull of gravity. Nevertheless, the bacteria are somehow able to swim in the correct direction to get back to the mud. How do they do it?
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