formally known as a raspberry, "thpt" is the phonetically translated form of the sound you make when pressing your upper teeth again your tongue, while sticking said muscle out of your mouth. after this is correctly arranged, rapidly pass air through the oral apparatus until a, in lamens terms, "fart" noise is achieved.
April: "SWEET! the Jonas brothers are coming to town!"
Ben: <sarcastically> "woo... THPT (n)"