Mexico's prickly pear cactus: energy source of the future?

The thick, spiny outer layer of the prickly pear cactus has always been a waste product -- until Mexican researchers developed a biogas generator to turn it into electricity.

The prickly pear cactus is such a powerful symbol in Mexico that they put it smack in the middle of the national flag. 

It was considered sacred by the ancient Aztecs, and modern-day Mexicans eat it, drink it, and even use it in medicines and shampoos.