The Knock app acts as a prompt for your absent-minded friends to answer your texts. The android application simulates the sound of an incoming call to get your otherwise-occupied friend's highly coveted attention.
When your pal fumbles to answer the call, he or she will see a call screen that features your text message. To answer, they simply swipe the screen to reply or indicate their location. Your pals will need to download and install the app themselves for it to work, a requirement which is limited by the fact that it's only available for Android user.
The next time you're losing a battle for your friend's attention (to a re-run of Friends, no less), simply use the Knock app to restore their focus to its rightful place: you.