Because that's all their tiny beaks can do...

Because they don't know the words.

Because they can not meow..:)

i don't know, but they're the most adorable creatures on the planet.

Crossing the road to get to the other side??

1) Not all ducks quack, and they produce also various other sounds. It is a kind of communication that their voice is able to produce. Humans could also quack. Ducks quack because "quack" is actually the name that we give to that typical sound produced by ducks. Further information: http://en.wikipedia.org/wiki/Animal_communication http://en.wikipedia.org/wiki/Bioacoustics http://en.wikipedia.org/wiki/List_of_animal_sounds 2) "Despite widespread misconceptions, only the females of most dabbling ducks "quack". For example, the scaup – which are diving ducks – make a noise like "scaup" (hence their name), and even among the dabbling ducks, the males never quack. In general, ducks make a wide range of calls, ranging from whistles cooing, yodels and grunts. Calls may be loud displaying calls or quieter contact calls. A common urban legend claims that duck quacks do not echo; however, this has been shown to be false. This myth was first debunked by the Acoustics Research Centre at the University of Salford in 2003 as part of the British Association's Festival of Science. It was also debunked in one of the earlier episodes of the popular Discovery Channel television show MythBusters." Source and further information: http://en.wikipedia.org/wiki/Duck#Communication

They're bored. Or they're arguing. Or they're scared. Or they want food. Honestly, I think the main reason is to annoy the humans. Edited to fix typo because it was driving me insane.

The fucking waters cold

because they have nothing else to do

Because he can say cock a doodle doo!!! +5

cuz they can't bark or meow or moo or baaaaaaaaaaaa

quackers go good with soup.

Cause they think their doctors.

It sounds like "quack" to us humans. That's how ducks communicate. If you listen carefully to this conversation between Donald Duck and Howard the Duck it might go something like this:   Donald: "Quack!" [Have you seen Daisy? That bitch was supposed to fix dinner for me! Where the hell is she?]   Howard: "Quack!" [I saw her over at McGregor's pond with Disco Duck. They were playing patty-cake]   Donald: "Quack!" [I'll kill her! I'll kill them both!]   You just have to listen closely.