Title text: I've looked into this, and I can't figure out a way to do it cheaply. And I guess it wouldn't be sanitary.
Randall is playing with the child/grownup setup again. During childhood, adults ("grownups") make most of the decisions and put constraints on what their children do. As children age and eventually become grownups, there are some things that they do not do anymore, as they see them as childish.
Megan has taken these thoughts seriously, and realizes that with her newfound freedom as a grownup, she gets to define what her adulthood means. Free from constraints, she goes ahead and creates a playpen in her apartment of colored plastic balls. Cueball admires this spirit and enters the playpen. It is difficult to decipher Randall's true intent behind this somewhat cryptic comic, but it seems from the heart that the two are making love in the balls.
The title text reveals that Randall also would like to make his own playpen, but he finds it expensive to buy and also hard to keep clean. Both of these are true - see links. Most public playpens can easily become very dirty as many children and toddlers play in them.
This is one of the five comics linked at the bottom part of the xkcd.com website.
- [Cueball is talking to Megan who is behind a waist-high screen across a doorway with colorful playpen balls behind her.]
- Cueball: Hey, I was wondering if you had plans for-- holy crap, what happened to your apartment?
- Megan: I filled it with playpen balls!
- Cueball: I... what? Why?
- Megan: Because we're grown-ups now, and it's our turn to decide what that means.
- [Both vanish into the apartment, balls spilling out into the corridor. A love heart drifts out the door, indicating you-know-what.]
- Randall, author of the comic, actually built one in his apartment, which he talks about and shows pictures of at here and here. Speaking of cost, chiliahedron.com has a calculator for how much it would cost to make your own ball pit.
- China takes the cake for this, though. China currently holds the record for the worlds largest ballpit: a swimming pool filled with 1 million balls.