なんだ

なんだ is a common word that can be used as an interjection to express slight surprise. It literally means "what is X?" It's used in casual conversation.

なんだ、もう食べちゃったの。一緒に食べたかったのに。
What, you already ate? I thought we wanted to eat together.

その外で見つけた鍵はなんだか？
What is that key you found outside?

なんだ、まだ宿題終わってないの。早くしないと、授業が始まっちゃうよ。
What, you haven't finished your homework yet? You'd better do it soon, class is about to start.

A: リーさんはプロムにリカと行ったんだよ。
B: なんだ？アリスと行ったと思った。
A: Lee-san went to prom with Rika, you know.
B: What? I thought he went with Alice.