A duck's quack doesn't echo, and no one knows why.
While both male and female reindeer grow antlers in the summer each year, male reindeer drop their antlers at the beginning of winter, usually late November to mid-December.
A chameleon's tongue is twice the length of its body.
According to the Alaska Department of Fish and Game, the pelican is the most powerful animal.
The woodcock can see in front and behind itself with both its eyes, effectively facing both backwards and forwards.
Hummingbirds can fly backwards.
UPDATE: It seems I must issue a correction...
Claim: A duck's quack doesn't echo, and no one knows why
Status: Partially true - see below
This is an uncontroversial fact dressed up as a mystery with the addition of the phrase "and no one knows why". We do know why, and have since 1950, when an aviator named Ellis Cornell decided to analyze the properties of bird calls using an oscilloscope. Cornell later patented a method for producing artificial "quacks" as a means of avoiding detection by sonar.
Quacks and other non-echoing noises have what acoustics experts call "negative envelope". Played backwards, a sound with negative envelope will phase itself out, like a double-sided strip of Velcro.
One mystery does remain: Why is this canard (pun intended) always about ducks? Their ability is hardly unique—almost all birds that live in aquatic environments have non-echoing calls, as do frogs, possums, and housecats.
Next time you hear someone pass along this "astounding fact", tell them to look up the northern pine snake—its "cough" is only audible on odd numbered echoes, and not when originally produced!