Split Hair

One way of showing the sharpness of a blade is dropping a strand of hair, or thread, on it and watch it split it in half. Not to be confused with splitting hairs, which is about fine distinctions.

Anime and Manga

 * Secret of Mammo

Literature

 * On Gor, blades are frequently described as being "so sharp it would cut a piece of silk dropped on it."
 * In Sir Walter Scott's The Talisman, Saladin demonstrates the sharpness of his Saracen sword by dropping a cushion onto it, which is neatly sliced in half.
 * Shows up in Frank Stockton's short story The Discourager of Hesitancy (a sequel to his more famous "The Lady or the Tiger?").
 * The split-a-piece-of-silk variant appears in Interesting Times, when a samurai attempts to intimidate Cohen the Barbarian.
 * Cohen then subverts it by throwing up a bogey-covered hankie, and decapitating three of them as they look up before they realize what's happening.
 * In the Hardy Boys mystery "The Secret of Pirates' Hill", Joe drops an envelope on the blade of some pirate cutlasses in the museum to test it's sharpness. {Hint, it's very sharp}
 * In Water Margin, Yang Zhi, one of the novel's heroes, is forced to sell his family heirloom of a sword in the street. When confronted by a drunken local bully who demnds a dirt cheap price, he tells the latter that the sword is capable of "cut metals like mud, slice through hair by the merest touch, and kill people with no blood on the blade".
 * In Squire, Raoul demonstrates the sharpness of Kel's naginata to Flynn by placing a feather on the blade. It, of course, cuts the feather in half without any effort.

Other

 * Here's a picture of St. Minutia.

Theater

 * In Sweeney Todd: The Demon Barber of Fleet Street, during the contest with Pirelli, Sweeney does this. Generally, on-stage, he pulls one of his hairs out, slices it, and watches it fall to the ground, all during a pause in Pirelli's song.

Western Animation

 * Ali Baba Bunny: Hassan manages to chop Daffy's hat, the diamond on it, and finally, the hair on his head.
 * I think Witch Hazel may do this in one Warner Bros. short. She certainly mentions that her blade is sharp enough to split a hair. Split a hare? EEEEE-heee-heeee-heee-heeee-heeee!
 * In The Adventures of Ichabod and Mr. Toad, Brom Bones intimidates Ichabod during his telling of the legend of the Headless Horseman by slowly splitting a hair off his head with a large knife.
 * Played straight and inverted in a Popeye short where Popeye and Bluto were competing store salesmen. Bluto shows that his knives are quality with this trope, then Popeye shows how crappy they are by having the hair split the knife.