Doc looks at his future hair dryer

Doc surveying the 1985 hair dryer he has found among his 1985 counterpart's belongings.

Darth Vader-Vulcan Salute

The same hair dryer doubles as "Darth Vader's" ray-gun.

"A hair dryer? Don't they have towels in the future?"
—Doc to Marty
" An hour later, after figuring out how the DeLorean time machine worked, Doc Brown took out the suitcase containing his 1985 articles and began examining them one by one. / "What's this thing?" he asked. / "A hair dryer," Marty said. / "A hair dryer? Don't they have towels in the future?" He tossed it back in the suitcase. "Don't tell me I'll actually use that," he muttered. "
—From Back to the Future by George Gipe (quote, page 125)

A hair dryer was an electrical device used for drying hair after washing.

Emmett Brown had packed one in his suitcase which he placed in the DeLorean time machine in 1985. When Marty McFly escaped from the Libyan terrorists, the suitcase with the hair dryer was transported to 1955.

When 1955 Doc found the hair dryer on opening the suitcase and Marty told him what it was, the scientist briefly wondered whether or not towels were still being used in the future. However, Doc was then distracted by the more interesting Playboy magazine — something which had just premiered in the world just two years prior — that was also in the case.

Marty would later use the hair dryer as a ray-gun in his "Darth Vader" persona.