- "He [Doc] stepped back to look at Marty's new outfit. "Not bad," he said. Reaching into the shopping bag, he pulled out a final purchase — a bottle of Vaseline hair tonic. As soon as he unscrewed the top, Marty curled his lip. / "Look, Doc," he murmured. "I'll admit that these threads are pretty cool. But you're not putting that greasy shit in my hair." / "Why not? A lot of the kids wear it." / "It looks terrible. And who knows what it contains? I mean, it might give me cancer." / "You need it for your disguise," Brown said. With that, he started coming some of it into Marty's hair. "Don't worry about it," he said. "This is supposed to be very fashionable, for both kids and grown-ups." / "Well, then, how come you don't use it?" Marty challenged. / "It's also very flammable," Doc replied. / "Great." "
- —From Back to the Future by George Gipe (quote, page 139)
- "Doc Brown stopped combing [the hair tonic into] Marty's hair, but seemed as if he wasn't quite finished. He looked sideways at the effect, not sure it was right. / "Allow me," Marty said, taking the comb. / Going to the mirror, he started combing the hair back along the sides and forced an errant curl to fall down across his forehead. / "If I'm gonna go through with this," he explained, "at least I'm gonna look like Elvis." / "Elvis? What's Elvis?" Doc Brown asked. / "You'll find out." "
- —From Back to the Future by George Gipe (quote, pages 139 and 140)
Vaseline hair tonic was a substance used to grease the hair of young adolescent males in the 1950s, creating fashionable styles.
Marty McFly was asked by Dr. Emmett Brown to wear some of this tonic to fit in with the crowds at Hill Valley High School. However, Marty was reluctant to do so at first as he didn't know what was in the tonic and that it might give him cancer. When Marty asked why Doc didn't wear it, Doc explained that it was "also very flammable" (see first Quote above).
Marty eventually decided that if he was going to have to wear the hair tonic, he was going to look like Elvis Presley (see second Quote above) and thus combed his hair into the style of the then 20-year-old and not yet famous King of Rock and Roll.