Considered one of the world's oldest mammals, the Opossum (aka Possum) has evolved relatively little over time. What's most likely kept the species alive and thriving down through the ages is ironically their ability to "play dead."

The defense mechanism possum employs distinguishes them from most other mammals and even their cousins, the marsupials. "Playing possum" is something these creatures will do in most life-threatening situations. Instead of fleeing from predators, possum learned a long time ago how to actually slow down both their heart rate and their breathing pattern so they appear dead -- only to "wake" once an attacker has lost interest and moved on.

However, that learning seems to be more involuntary than planned - something that's become become part and parcel of their DNA, more than a conscious on-the-spot decision. The scientific root cause points to possum actually going into shock when particularly stressed. In this unconscious state, their bodies go limp where their legs fall out from under - comparable to a human fainting.

According the Wildlife Center of Silicon Valley, playing possum is actually the animal's last resort to fend off threats. Considered non-aggressive -  even though its 50 razor sharp teeth might indicate otherwise  - if a snarl or hiss doesn't ward off the predator, possum are known to drool and blow bubbles through their noses, to appear diseased and unappetizing.

Dogs, one of their major predators, are dissuaded from attacking them when they encounter a live, growling animal that falls over, becomes stiff, rolls up its lips, starts oozing saliva and exudes a very foul odor from its anal glands.
Short-tailed PossumShort-tailed Possum
Short Tailed Opossums are growing in popularity as an exotic pet. They're cute enough but maintain a proclivity for trying to escape. Pet owners suggest an aquarium or narrow-mesh wire cage to keep them home on the farm.

Some have even been known to perk up when they hear music. This cartoonist interpreted this type of behavior as such...

Possum playing The Dead!Possum playing The Dead!
While best kept as solitary pets, a tame, often-handled young possum (3-4 months) will sometimes open up to the company of other pets - and manifest less of a tendency to play dead. That is unless their being filmed for a Geico commercial. Then you can bet, they're going to show the world their acting chops and how best to "die on cue," -- only, to scare the pants off of you when they revive!