Imagine trying to explain that you should be able to build a nuclear reactor or a moon rocket to someone who has never heard of physics, in 1920 when you don't have the tech to do either yet.

H. G. Wells did it: http://en.wikipedia.org/wiki/The_War_in_the_Air http://en.wikipedia.org/wiki/First_Men_In_The_Moon

Also, people can sometimes do it themselves:

http://www.smithsonianmag.com/history-archaeology/For-40-Years-This-Russian-Family-Was-Cut-Off-From-Human-Contact-Unaware-of-World-War-II-188843001.html

Relevant quote:

"As the Soviet geologists got to know the Lykov family, they realized that they had underestimated their abilities and intelligence. Each family member had a distinct personality; Old Karp was usually delighted by the latest innovations that the scientists brought up from their camp, and though he steadfastly refused to believe that man had set foot on the moon, he adapted swiftly to the idea of satellites. The Lykovs had noticed them as early as the 1950s, when "the stars began to go quickly across the sky," and Karp himself conceived a theory to explain this: "People have thought something up and are sending out fires that are very like stars."