Taking advantage of iPhones advance sensors, surprise your friends with this fake fart detector:)
When you press "Start" to detect, the touch point is where the detector will point to. e.g. if you touch the top part of the "Start" button, the detector will point at the front location. If you touch the left part of the button, the detector will point to the left direction. The best part is that the direction is locked. Even if you rotate or move your iPhone, the pointer will keep pointing at the same direction, which makes this detector much more convincing!
Have fun!