Often used in such cartoons as the famous "Calvin and Hobbes" to illustrate a farting noise made with the mouth. It is also the best way to get whoever is talking to you to stfu.
Steph - Liek omg i today i wuz awl liek deezinarr perfyoome shawpeeng wen dis hoez cumz all up in mah grill liek OMG can i haff ur autograf plz?!?! s o i r just liek mmkah cuz i r prettier than u r lolololol
You - PTHLTPBTH!!!!!!!
Steph - ... LIEK OMG SO IMMATU
Steph - ... *Storms off muttering about how much better than everyone else she is.*