Very quickly, Lan Wangji opens the bottle and tries to capture the laugh, whatever that means. His eyes widen the slightest because he hadn't expected that to work, he didn't think it was funny. He had just overheard this one day and contextually understood it as a joke.

Before Sizhui can say anything else, he marches back over to the vendor to make the trade, and comes back with the butterfly, offering it to Sizhui with a very serious look on his face, as per usual.

They can just never mention the joking.