But before that, Ernst Jentsch wrote about "the uncanny" in a 1906 essay, and Sigmund Freud followed up 13 years later.

CNN: Why zombies, robots, clowns freak us out

"We have in the safe a lead lined container that contains some of the original radium that Madame Curie possessed, " says Jentsch.

NPR: Hidden Museum Treasures

While discussions of cultural differences can spark debate, the Smithsonian's Eric Jentsch, a specialist in the NMAH's division of science, medicine and society, points to one object that's literally too hot to handle.

NPR: Hidden Museum Treasures