Why is the sentence like that?

[Ivan978321]

Hi everyone!
I started lerning german recently and i saw the following sentence in my book:
"Ich habe einen großen Kopf"
Meaning - I have a big head

My question is that why is the adjective conjugated in accusative and not in nominativ?
Why isnt it: Ich habe ein großer Kopf?

I hope someone can explain it to me:)

[Raisinnoir]

Because Kopf is the object and adjectives in German must agree with the case of the noun.

[Ivan978321]

Ahh its clear now, thank you!:)