Tripwire Hook

A Tripwire Hook is a type of switch that can be placed across a gap for the player to activate. They are activated by the player stepping on them or destroying the wire while not holding shears. Tripwires are one of the five types of switches to be introduced into the game.

Usage
The player first attaches hooks across from each other at most 18 blocks apart. Then, the player must place string across the gap between them. The string can "float" across this gap, not requiring blocks beneath it to be placed. Once connected, the hooks will lower. At this point, walking on, or destroying the string (without shears) will cause redstone current from both hooks. The string must lay in a straight line between the two hooks, or else it will not not activate when someone steps on it.

History
On May 28th, 2012, Dinnerbone began tweeting of a new idea he had. He began posting pictures and eventually released a video on Tripwires. Before this, the player could not place string on the ground. Dinnerbone credited the idea to his fiancee MissMarzenia.

Trivia

 * Although placing String on the ground is mainly for creating Tripwires, String does not need to be used in this way and can be placed anyway the player likes.
 * Tripwires can detect players in minecarts if they're placed over tracks.