No rule for this one
No ending or word group predicts the article of Ei. Learn it as a pair and say it out loud: das Ei.
das Ei
It's das Ei. Ei is neuter, plural die Eier.
Zum Frühstück esse ich zwei Eier.
For breakfast I eat two eggs.
No ending or word group predicts the article of Ei. Learn it as a pair and say it out loud: das Ei.