And voilà, here it is and as simple as it gets. You may call it a letter-scroller/shifter/slider/spammer/whatever, I just named it HeyReadis, but just to make it crystal clear, here are the steps:
1) Type a message
2) press Display
3) point the screen to someone
4) say "hey read this!" (or just call their attention, but it will be way cooler to say the app name instead ;P)
Letter by letter the message shows up in full-screen changing at a constant rate. If that someone doesn't get it, you can press the back button and type a new speed (changing rate in milliseconds) to more acceptable or challenging levels. You may also use a group of blank spaces to delay a new letter.
Test your friends and your reading ability (by setting the speed to 0 milliseconds!!!). Have fun.