So hot! But I learned a new Japanese proverb: Natsu no mushi, kōri o warau, "Summer bugs laugh at [the idea of] ice." Use it to describe someone who knows less about the world than they think.

The source is interesting: it comes from Zhuangzi 荘子, from the "Floods of Autumn" chapter. In the original and James Legge's translation:

北海若曰、井蛙不可以語於海者、拘於虛也。夏蟲不可以語於冰者、篤於時也。曲士不可以語於道者、束於教也。

Ruo, (the Spirit-lord) of the Northern Sea, said, "A frog in a well cannot be talked with about the sea - he is confined to the limits of his hole. An insect of the summer cannot be talked with about ice - it knows nothing beyond its own season. A scholar of limited views cannot be talked with about the Dao - he is bound by the teaching (which he has received)."

Yes! Zhuangzi is also the source for the famous "frog in a well" analogy (meaning basically the same thing). Although he used that one more than once.