What you're asking is probably the most difficult point of English grammar. I don't think there's a rule saying when to use a possessive (my mother's house), an objective (a table leg) or a structure with of" (the north of Edinburgh). In my opinion, non-native speakers are quite good at this once their level of English has become advanced.
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Siernando you think if i save the 's for people and animals i will always be correct?
Siernanthe tip of my tongueThese are both correct as you have written them. "my tongue tip" doesn't sound natural, but "the tips of my fingers" is fine.
but
my finger tips