An external trigger communicates the next action the user should take in an explicity clear way. They are:

1. Paid triggers-Ads.
2. Earned triggers-Press, viral videos, App Store rankings, SEO.
3. Relationship triggers-Referrals and word of mouth.
4. Owned triggers-A part of your user's environment by their own choice: the app icon on the home screen, an email newsletter, or push notifications.