I think it’s a fun question. I’d argue that sanity doesn’t exist, as the concept of sanity is constructed entirely of opinions–if a concept is made up of opinions, then the concept itself is an opinion, and is therefore entirely hypothetical.
How do you know you’re in your right mind?
Psychology defines it in terms of cultural norms and whether it is distressful or harmful for the individual. Are you exhibiting behaviour outside of what your society deems normal? Sanity will be suspect. Does your behaviour bother you, seems to be outside your control, or is harming the quality of your life? Probably can be diagnosed as some psychiatric malady of one sort or another.
The example I recall in class is a culture in which witches talk to spirits. Witch comes in alarmed that they are talking to spirits all the time and can’t stop. Psychiatrist asks ‘is that not your job?’ and they reply “yes, but I’m talking to the wrong spirits”.
If your question was how to use a metaphor to describe sanity, anything that represents stability is common (“even when the shit hits the fan she is solid as a rock”).