"What do flies eat?"

Translation: Co žerou mouchy?

[LoossSS]

I wrote "Co mouchy žerou?" and it was marked wrong.
How could I know it's not the good word order ? Is there a trick here or I just need to know it by heart ?
Or it's not wrong, but here we want to emphasize that we want to know what FLIES are eating, so we put flies at the end ?

[VladaFu]

added

[ali940239]

Co jedí mouchy......mi neuznalo. Musí být nutně výraz"žerou"?

[VladaFu]

Zatim musi, ale jedi asi pridat vyhledove muzeme. Prijde mi to detinske, ale je to asi mozne. Nicmene - chybejici preklady hlaste tlacitkem, jinak to tam neuvidime a nic nepridame.