Opuntia demand is making cultivation of the insect an attractive opportunity in other regions, such as in Mexico, where cochineal production had declined again owing to the numerous natural enemies of the scale insect. Apart from cochineal, the red dye betanin can be extracted from some "Opuntia" plants themselves. Fluid ("cactus juice") extracted from "Opuntia" pads and stems, especially "O. ficus-indica", is one of the most commonly used additives in earthen plaster. The flesh ("mucilage") of the cactus has been found to purify water. A project at the University of South Florida is investigating its potential for low-cost, large-scale water purification.
What Is and What Should Never Be descending riff [of "What Is and What Should Never Be"] is amazing: It's like a bow is being drawn back, and then it releases. The rhythm of the vocals is almost like a rap. It's insane — one of their most psychedelic songs." This was also one of the first songs recorded by the band for which Robert Plant received writing credit. According to rock journalist Stephen Davis, the author of the Led Zeppelin biography "Hammer of the Gods: The Led Zeppelin Saga", the lyrics for this song reflect a romance Plant had with his wife's younger sister. "What Is