LSD first became popular in the 1960s. A Harvard psychology professor, Timothy Leary, began to encourage people to try the drug then. College students willingly took part in LSD experiments. These experiments were made by psychologists and other professionals. Leary and two of the people he worked with, Richard Alpert and Ralph Metzner, thought the transforming effects of the drug could be a kind of 'rebirth' of users, in the same way as many religions offered them. They wrote a book, "The Psychedelic Experience", which was based on the Tibetan Book of the Dead. Leary eventually became well known in the 1960s hippie movement for his slogan about LSD: "Turn on, tune in, drop out". The Hippies were a countercultural movement. A number of famous rock bands, including the Beatles and the Grateful Dead, became known for their use of LSD, and even a new type of rock music, called "acid rock", was born from the fad.