When your self goes down a slide and hits a barb, it's a painful reminder that you have a body. Sometimes people think their self is just some kind of floating, disembodied consciousness, but getting a barb stuck in oneself while going down a slide will quickly remind one of one's bodily existence.

What part of your body did the barb get stuck in? Your shin (しん). That pointy little barb dug deep into your shin and made you painfully aware of the fact that oneself and one's body are inextricable.