A common problem with older cars is that water can collect in the compartments that house the the headlights, sidelights, and the turn signal lights. Although the compartments are often nearly air tight, water seeps in very slowly over time. The simplest remedy is to drill a small hole near the bottom of the lens so the water can drip out. (That's usually a better solution than spending many wasted hours trying to seal the comparments.) Below is described an actual ocurrence.
The water was about an inch deep in one of the lenses, and the lens was clear above the water line. A hole was drilled, the water started to drain, and it was noticed that the lens above the water started fogging over. By the time the water had flowed out, about a minute later, the lens was completely fogged over on the inside.
Can you explain why this happened? Here's a hint. As the water drained out, air could not immediately flow in to take its place. So the dropping water level actually decreased the pressure of the air inside the comparment.
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