Install "Surprise" in your friends's devices secretly. Then you can use "Surprise" to push photos to scare him !!!
Steps: 1). Install "Surprise" in your friend's device. 2). Click "Act as victim", remember the "Surprise ID", then click "hide app", and give it back to your friend. 3). Install "Surprise" in your device. 4). Enter "Surprise ID", and click "send" button when your friend's playing phone game. 5). Your friend will be scared, and he may kill you.
Technical features / details: 1). Use google app engine ( so don't blame me if you feel it's too slow ) 2). Use google GCM 3). Use NFC technology ( push "Surprise ID" )
Have fun ! Comments/feedbacks are welcome, but don't expect me to reply, as I'm quite busy usually hahaha.