Stuart was feeling "chirpy" as he watched two squirrels in a walnut tree from his bed. Because Susie's mind was typically in the gutter, she assumed this to be a euphemism for the contents of Stuart's underwear. If only she had double carpeted her bottom step!
What happens when you are driving, and a squirrel suddenly darts in front of your car. As you swerve to avoid hitting it, the squirrel also swerves, right under your front tire.