A photography problem
Joke:What is a joke?
"Doctor, Doctor My little boy has just swallowed a roll of film!"
"Hmm. Let's hope nothing develops. "
A roll of film here means when you put in a camera to take photographs with.
The joke here lies in the the word develops. When you take photos, you then get them developed in a photo shop. However, in medicine, when an illness develops, it becomes worse.