system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why when the bathroom mirror fogs up does it “defog” itself from bottom to top and not top to bottom?
gpt
Remember the old expression, "Heat rises"?   Well, that means that as the air cools back to "room temperature" in a bathroom, it appears to do so from the floor up.  That's because the cooler air is dropping to the floor, pushing the still warmer air to the ceiling.

Cooler air is able to absorb more heat from other matter, like glass, than warmer air.  So the lower, cooler air is able to draw away heat from the portion of the mirror that it touches, which is near the bottom.  That air then becomes warmer and rises again, pushing now cooler air down, and continuing the process.