In episode 1 at 12:36, right after the scene of the principal's bad jokes (which is the subject of this question: How do principal Shinonome's jokes in ep1 work (what's the joke), and what could they have to do with the meaning of hard work?), Mai discovers a squid tentacle in Yuuko's hair and laughs at it with tears coming from her eyes. Yuuko thinks Mai is laughing at the principal's bad jokes, and seeing that Mai is laughing more than she has ever seen her laugh, sees it as an opportunity to try cracking a joke of her own. She then makes two closed hands except for both her index fingers pointing upward, and her right hand holding her left index finger and tells Mai to look.
Here are some screenshots:
What's supposed to be funny about this here?
Is there some cultural reference I'm missing? Is the joke building on anything to do with the principal's previous jokes?
My intuition tells me that there's actually a meaningful mechanism to the joke here that I'm just unable to understand as an English viewer without anything more than shallow knowledge of Japanese culture.
I know that absuridty ("randomness") and non-sequiturs are often used as a mechanism for humour in Nichijou, but from what I remember, Yuuko's efforts to make her friends laugh are honest attempts at humour. Absurdity is more Mai's thing. Yuuko does use non-sequiturs in her poetry, but that's different. Yes, the joke here falls flat- as if Mai doesn't understand it (the question mark that appears above her head), but so do most (all?) of Yuuko's attempts at making her friends laugh through jokes.