Why did the tourist bring a ladder to the art museum?

This joke gets its chuckle from a clever bit of wordplay, specifically a pun that plays on the double meaning of the phrase "through the roof." When we hear that something, like reviews for an art exhibit, is "through the roof," it's a common idiom meaning the reviews are exceptionally good, overwhelmingly positive, or incredibly high in praise. It's a way of saying the quality or reception is outstanding.

However, our tourist hilariously takes this idiom quite literally. Instead of understanding that the reviews are just excellent, they picture the praise actually breaking through the physical roof of the museum, perhaps imagining they'd need a ladder to reach such elevated acclaim. The humor comes from this absurd misunderstanding, contrasting the figurative meaning of the idiom with a silly, literal interpretation that conjures up a funny image of someone scaling a museum wall just to read a glowing critique.