Seismograph
Permanent link to this comic: http://xkcd.com/711/
Image URL (for hotlinking/embedding): http://imgs.xkcd.com/comics/seismograph.png
[[A man is standing over another man, who is strapped into a chair with wires attached to his head and arms. The wires lead to a large lie detector on a stand next to him, which has jagged lines drawn across it.]]
Standing man: IS THERE AN EARTHQUAKE HAPPENING?!
Sitting man: No!
<<SCRITCH SCRITCH>>
Pro Tip: In a pinch, a lie detector can double as a seismograph.
{{Title text: The reverse only works if the subject has a nervous twitch.}}