This is a prank you can play with friends who use iMessage (you need to be using iOS devices that can support iMessaging).
Watch this video, then download the gif from here and save it on your iphone.
Download gif here
Essentially, you select a friend whom uses imessage, attach this gif that you have downloaded and send it to him/her via imessage, without any text. It works best when you are in the midst of an imessage conversation with them and you know they are online / checking their phone.
This is how the gif file looks like