No rule for this one
No ending or word group predicts the article of Kopf. Learn it as a pair and say it out loud: der Kopf.
der Kopf
It's der Kopf. Kopf is masculine, plural die Köpfe.
Mein Kopf tut weh.
My head hurts.
No ending or word group predicts the article of Kopf. Learn it as a pair and say it out loud: der Kopf.