Anyone remember doing the below in MS Paint? Chances are, it was probably one of your early introductions to digital graphics.
You can do the same thing with the Live Paint Bucket (K). In fact, you could do the same as above with the Rectangle Tool (M) and the Live Paint Bucket.
One of my favorite tricks is using it to fill in a “pixel”-like grid, like the below end result. However, there is plenty more you can do. Filling in line art with this tool is a great time saver!
There are of course many great uses of this wonderful tool, so allow me to recommend some great tutorials.