il labbro e le labbra
il labbro (lip) / le labbra
il frutto (fruit) / le frutta
il braccio (arm) / le braccia
il corno (horn) / le corna
il muro (wall) / le mura
So do their grammatical genders change from singular to plural, then, like so:
il labbro piccolo le labbra piccole
Yes, that's exactly what happens :) Some more examples are l'uovo -> le uova, il lenzuolo -> le lenzuola and l'osso -> le ossa. It's not very common, though.
Uhm... "le frutta" isn't wrong, but is defenitely uncommon.
It's usually said this way:
"i frutti" = many specific fruits, also - along with the singular form "frutto" - with figurative meaning as in "results" "la frutta" = as in the uncountable noun "fruit" meaning food, collectively speaking.
L'arancia è un frutto = Orange is a fruit.
Hai mangiato la frutta? = Did you eat fruit?
Questo è il frutto della maldicenza = This is the fruit of malicious gossip
L'azienda ha saputo curare i suoi clienti, e adesso ne raccoglie i frutti = The firm took good care of its clients, and it's now reaping the benefits.
Hope this might help. :)
It is the same with "le mura" as they refer to city walls and not the walls of a house AFAIK. Let me know if I am wrong :)
Curiously, changing gender when growing up is not exclusive to Italian words, but it is observed in animals too and called "Sequential hermaphroditism" http://en.wikipedia.org/wiki/Sequential_hermaphroditism Same in plants...