user
Given a cartoon image and a caption. Caption: So, when did the 'check mustard' light come on? Question: Generating an explanation of why this image/caption combination is funny. Answer: 
assistant
A normal vehicle that breaks down might have the "check engine" light on, which a mechanic might ask about the timeline of. Here, the car is the Wienermobile, which might instead have a "check mustard" light instead. It's funny partly because it's unclear what the check mustard light would indicate about the hot dog repair.