Glass isn't really a solid: It's actually a super-cooled liquid, and it flows over time - very slowly, but it does flow. As a result, 100 year old vertical panes that were once flat sheets now have noticeable ripples and variations in thickness - so the view through old windows is somewhat distorted, much like our recollection of past events. I remember when I took this picture, but the memory is rather like the view out the window - it doesn't quite match what was there ...