Calm Lakes
I always like to get a picture of some mountain range reflecting in the mirror surface of a mountain lake. Particularly at sunrise or sunset. In my opinion, these make some of the most interesting photographs. But why isn't the lake surface calm during the day.
It has to do with temperature and wind. Wind is what causes waves on the lake surface. Temperature difference is what causes the wind. As the sun begins to warm up the surrounding area wind is produced thereby causing waves. This is a simplistic explanation but sufficient for my needs.
Generally, the area around the lake is cold in the early morning while waiting for the sunrise to hit that mountain range just right!