If you love your girlfriend you should never do this prank to her–more than once. Once is enough–that’s a true sign you really love her.
You see, the point is to make her naturally hug you without you asking her. Natural show of love sometimes needs a bit of stimulation or encouragement and technology can be a great help in this. The video here shows a realistic way how. Very convincing.
If you’re no techie, then a simple Boo! would do. Well, just to give you an idea, watch the video.