While Arnold is in disbelief over the fact that he just went on a completely, normal, mundane, commonplace field trip to the zoo, Tim agrees and theorizes that seeing wild animals would have been much weirder if they, oh, saw them wandering around the city or something...
Tim: Can you picture that?
Ms. Frizzle: Absolutely, Tim! We can all feature wild creatures!
Arnold:What?! Tim, quick — take it back!
In "Cracks a Yolk":
Arnold: Why oh why do we always wind up in long, dark, slippery tubes?
Ms. Frizzle: Oh, we're just lucky I guess, Arnold.