You can't run through a camp site. You can only ran, because it's past tents.

Behind the Groan

Running Into the Past Tense

“Past tents” sounds like “past tense,” the grammar category that changes “run” to “ran.” The sentence deliberately uses the wrong grammar so the camping location can dictate the verb. Read the first sentence like a serious campground rule. Pause before “You can only ran,” letting the obvious error stand out, then finish with the explanation. The joke lands when listeners connect the tents being passed with the grammatical reason for changing the word.