Funny! Jokes! And Tricky! description

This is a most popular and downloaded Tricky Software. After starting the application effect,put the phone into the pocket of somebody, for a while, a fart will be explode (of course, is the only sound, no smell of absolute environmental fart). Looked at their flushed, tense and awkward look, you will be the United States doubled. \ N
The method of operation is very simple, try to find it.