All week, the comic strip "Non Sequitur" has been running gags about whether or not a duck's quack echoes. The joke is that once someone asks you the question, you can't stop thinking about it until you know the answer. It's Friday, and I've resisted the temptation to look it up until now, but I've caved!
I can't think of a single scientific reason why a duck's quack WOULDN'T echo, but I had to look it up anyway. The good news? I'm hardly the first person to do it. When I googled "Does a duck's quack echo?" I got 105,000 hits, including links to some real research.
Here are some of the best sources of info:
Salford University: "The duck's quack echo myth" (This is an awesome page.)
SPOILER: Yes, a duck's quack, like any other sound, echoes. But the WAY a duck quacks, with the long "AAAAAACK" sound at the end of the call, tends to mask echoes, making them hard to hear.