The accusative case makes the object a direct object where you are going to do something with that object.
In other words, the direct object is "receiving the action of a transitive verb". (1)
What can you do with that object? Almost anything. You can see it, you can touch it, you can cut it, you can eat it, you can throw it, ... It depends on that object itself.

Ali, once explained the transitive verbs with a tomato example which was an interesting reading. Maybe you can find his message.

Of course the object that takes the accusative case suffix wont be a subject of any clause or sentence.

(1) Quote from this page:
http://www.answers.com/main/ntquery?sm1=ZGlyZWN0IG9iamVjdCA=&fw=-1&fc=-1&ss=0&es=0&gwp=11&ver=1.0.8.207&method=1

Yeah, if you meant dog food than köpek yemeği would suit better but the problem is that there is no common agreement on such a usage and köpek yemeği sounds too unusaual for me. I will give you alternatives:

dog food:
1. köpek maması
2. köpek yemi
3. köpek yemeği


While number three is very uncommon, number two is more common but the strongest tendency is to use köpek maması. I just checked this and it seems köpek maması is currently the what people use.
http://www.garantialisveris.com/petschoice/str_prod.asp?storeID=3172&productID=85519
http://www.pet.gen.tr/2002/alisveris.phtml?mid=200&cid=17
http://www.zureyfa.com/Shop/dept.asp?DeptID=20102&page2go=4

My pronounciation is not so good compared to my level at writing and reading in English. I think the reason is the stress on words. I just can't stress the words as a native English would do.
Even the simplest word like hello sounds still too complicated to me even tought I'm living in the UK for a year and a half now.
I think this is because we don't stress words as much as the English speakers do. The stress in Turkish words is much less strong than in English words. It would be alright if you don't do any stres at all. On the other hand there is the rytm of pronounciation which is a little different I think. It is like sound waves going up and down. Ecpecially we see the rytm with the question suffix.

Yes your translation is acceptable but it sounds to harsh to me compared to silly. Aptal is more like stupid. Silly, on the other hand is more friendly and funny.

I have seen moms calling their kids as "silly boy", or "silly girl". Maybe one of these would suit better:

sersem, ahmak, şapşal, budala, maskara

I would pick şapşal if I had to pick only one word.

Hi sammyscuba,
there are tiny mistakes on two answers.