I really respect people who can do that, though, so when the time-travelling salesman came back today, I was eager to give him a chance.
He was selling shampoo from the future. Nanobot shampoo. A technical marvel. The tiny nanoscopic robots removed all buildup from the hair, sealed in moisture, and left hair full, manageable, bouncy and shiny.
"How much does it cost?" I asked skeptically.
"Three hundred dollars for the bottle," he said, "but it's self-replicating. When it's half-empty, just fill it with water and drop in a slice of raw bacon, and the nanobots will make more."
"Why didn't you bring back something more impressive, like free energy or superweapons or perfect computers or something?" I asked skeptically.
"I didn't want to attract a lot of government attention," he said, "There's a lot of people who'd try to get me if they knew I traveled through time, but something like shampoo sales will go under their radar. I hope."
"Okay, then why sell door to door?" I asked skeptically.
"I was a door-to-door Fuller Brush salesman before I was a time traveler," he said, "It's the way I like to do business."
"Okay, fine. Believable, demonstrably perfect product, amazing price. I have one last question."
He smiled confidently, "Ask away."
"This shampoo. Does it have a luxurious rich lather?"
He hung his head low. "No, it doesn't lather at all. Everybody in this time asks that. Why does it have to lather? Isn't perfect hair enough?"