1. When the platypus was first examined by a scientist, the researcher thought he was being pranked, saying "of all the Mammalia yet known, it seems the most extraordinary in its conformation; exhibiting the perfect resemblance of the beak of a duck engrafted on the head of a quadruped... I almost doubted the testimony of my own eyes with respect to the structure of this animal's beak."
2. The platypus, which hunts in water, finds prey by solely picking up on electric currents received through its bill. From these signals, the platypus can hone in on its prey. A platypus' bill is so sensitive, it can even sense a small battery at the bottom of a river.
3. Male platypuses are venomous. They have a small spur on each rear ankle, which produces a venom that is comprised of molecules called defensin-like proteins. This venom is incredibly painful, though unlike the toxins from many venomous creatures, it won't actually kill -- it will merely incapacitate.
4. Platypus females do not have nipples. To suckle their young, platypuses have milk patches similar to areolae that produce milk. The female platypus secretes the milk through her pores, in a manner very similar to sweating.
5. The platypus is the only remaining animal (besides the echidna) that is classified as a monotreme, or egg-laying mammal. The platypus lays her eggs, and then incubates them by curling around them. The babies hatch after about 10 days of incubation.
6. A platypus doesn't really have a stomach. Instead of a separate pouch where food collects, the platypus' esophagus is directly connected to its intestine. There isn't much knowledge about why the stomach was eliminated over time, although there is speculation that the simplicity of the platypus' diet lead to this adaptation.
7. Platypus don't have teeth -- when they need to break down food, they improvise. These billed creatures eat larvae and worms, but occasionally a shellfish or crustacean will catch their interest, so they'll need outside assistance to break apart the tasty morsels. To accomplish this, a platypus gathers gravel in its mouth along with its prey, and mashes the food into manageable pieces.