The Google Earth Blog is a great place to find interesting and educationally useful KML files. Today, on the Google Earth Blog, a KML file that displays and automatically updates the where in the world it is night or day. The updates occur every thirty seconds. The file also includes a layer displaying and automatically updating cloud cover around the world. You can access the file here.

Applications for Education
This KML file could be useful for showing students how daylight changes as the world rotates. The file could also be useful for science teachers teaching meteorology lessons to let students observe shifting cloud patterns.