It's a genetic disease

Where your shoulders go straight down to your ankles.  Another thing he gets from his Dad.

 One of Lex's favorite stories is Dr. Suess' "What was I afraid of?"  Which he calls, The Scary Pants Story.  It's a story about someone who comes across a 'pair of pale green pants with nobody inside them'.  There's a line from that story that is the perfect caption to this picture. 

"Poor swimtrunks with no butt inside them!"