I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
Take an inflatable swimming pool, let it sit outside. Notice any tide? No? Well, why would your kidneys have tides then?
Then why do I pee more when the moon is full, huh? Checkmate, atheists!
/s
Fine, I’ll admit there’s a god… just put away the moon pee!