They actually did this one on Mythbusters. As it turns out, a duck's quack has an odd waveform that tends to cancel itself out better than a lot of other sounds. What actually happens is that an echoing quack sounds just about like a non-echoing quack - that is, a duck's quack sounds like it's echoing even when it isn't, so if you're familiar with what ducks sound like, you won't notice it echoing.
Actually, don't read the above. It's terrible. Instead, read the synopsis at Wikipedia#Does_a_Duck.27s_Quack_Echo.3F "Duck's Quack episode of MythBusters")
Dale McGowan writes:
In this spirit, I propose the creation of a fully off-topic discussion thread.
Here is our monthly place to discuss topics entirely unrelated to Less Wrong that (of course) have not appeared in recent posts.
ETA: There are two behaviors I would love to see associated with this thread. First of all, discussions often drift off-topic in the middle of a thread. In these cases "let's take this to the off-topic thread" would be an excellent response. Secondly, given who's doing the discussing, I could easily see, say, a discussion about recent developments in some webcomic blossoming into a LW-worthy insight, in which case someone could spawn a new thread.