The mirror test is a measure of self-awareness. Basically, if you can look in a mirror and understand that the reflection you see is in fact you, then you pass the test.
Only a select list of species–elephants, dolphins, magpies, gray parrots, some primates, and humans–can pass the mirror test. As such, scientists regard the ability to use a mirror as evidence of complex cognition.
Miss Piggy, the famed porcine muppet, knew a thing or two about mirrors. In fact, she was seldom without one. She may have been vain, but she was also one smart pig.
Now, it turns out, Miss Piggy isn’t the only clever porker. Real pigs also understand the value of their reflection, they can pass the mirror test, thus putting them into the elite group of animals. Read more here.