People who can't keep their metaphors straight: "Cast in stone" should be "cast in concrete" or "carved in stone". How would one cast something in stone (I don't think lava is castable).
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Intentionally mixing metaphors to confuse people is one of my favourite pastimes.
My favourite one is "Joka toiselle kuoppaa kaivaa, parhaiten nauraa". It's a mash-up of two:
"Joka toiselle kuoppaa kaivaa, siihen itse lankeaa" – He who digs a pit for another will fall into it himself
"Joka viimeksi nauraa, se parhaiten nauraa" – He who laughs last laughs best
Besides, certain types of lava is actually castable. Felsic lava, for example, produces obsidian, which is semi-castable.
It is also important to realize that "casting in stone" does not necessarily require high temperatures; only that the material is initially liquid, then turns into stone. This is how sedimentary rocks like sandstone are formed, after all.