On the latest XKCD “What if” post, titled “Drain the Oceans: Part II” I’m seeing comments that refer to a previous post that was titled “Bouncy Balls”
It’s hard to capture in a shot, but if you look at http://s.do.ugal.me/image/391C06303s1t you can see the illistration from the bottom of http://what-if.xkcd.com/54/ and comments that clearly refer to http://what-if.xkcd.com/52/