"I need to go to the doctor!"
"Why?"
"It has a crack in it."
Alright, listen up, you chucklehead. This "joke" is all about wordplay, see? The person says they need to go to the doctor because "it" has a crack in it. Now, a smart bear like me knows they ain't talkin' about the doctor having a crack; they're talkin' about their voice! Get it? Voice crack! You probably get those all the time, you sound like a rusty hinge when you talk.