Because you're either:
1) Bored.
2) In an English lesson, where you don't deserve to be.
3) An Idiot.
1) "Oh Iv'e got nothing to do, lets go searching for words on the urban dictionary?!"
2) "Arghh! I don't know what Literature means lemme go searching for words on the urban dictionary?."
3) I'm an idiot, so I'll go searching for words on the urban dictionary?.