I think my friend did that more often; I was in the habit of having graph paper at hand, myself. There was also a lot of little flowers and dogs and stuff like that, sometimes even very sophisticated pictures. And we exchanged doodles, or one of us would start something and the other would finish it.
Somehow, I even managed to occasionally write a story under the table one year. Nowadays, I have no idea how I managed that. I could use more of that ability to squeeze in some writing!