This is a good way to not only use all of that leftover yarn from projects, but also make a very thick and warm lap blanket. (I have also done this using two strands of colors to make an actual pattern.)
I started out with two strands of different colors from my yarn pile and chain stitched 200. Then it was back and forth with a double crochet until it was the size I wanted. Each time one color ran out, I'd grab another out of the box, not caring for color match or size of the ball. It was all very random but makes for a great pattern. This one is used by my 3 year old and it is the perfect size to cover his whole body and will be a great lap blanket once he gets older. It has also been washed at least a dozen times and holds together very well.