About 16 years ago we moved into a house on the Delaware River waterfront. I had noticed that the water in the toilets would occasionally be at a lower level than after the toilets had refilled, and I was concerned that there was a leak. Then, one day when it was especially windy outside, I noticed movement in the toilet water, and I remembered the Bernoulli principle, reasoning that the wind blowing across the vent pipe on the roof created a vacuum, which would suck the water out of the toilet.
Mystery solved.
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