Glass Is a Slow-moving Liquid.
You may have lived for field trips as a kid, looking forward to a whole day of out-of-school fun and exploring. That is, until you got started on a tour of some musty building that seemed, well, boring. Not even the tour guide's explanation of how the glass in the wavy, uneven windowpanes has slowly flowed downward over time could keep your attention.
Liquid windowpanes? No.
Rather than the (magical-sounding) slow drip of centuries, the reason old glass windows aren't perfectly even and clear is because of how they were made. Until the early-mid 1800s, most window glass was made using a process called the crown method. The glass was blown, flattened, heated and spun, yielding a sheet that was relatively cheap to produce. It was also rippled and thicker in some places than in others.
In other words, the windows looked that way when they were installed, and they look that way now. No downhill liquid flow is involved. (And if you're really wondering: Glass is an amorphous solid. Learn more about it in "What makes glass transparent?")