The document discusses the 90/10 principle, which states that 10% of life is made up of external events outside our control, while 90% is determined by our reactions to those events. It provides an example of how a spilled cup of coffee could lead to a bad day if reacted to negatively through anger and scolding, but may be avoided by responding to the situation calmly. The key message is that while we cannot control what happens to us, we can control how we react, which largely determines our experiences. Applying the 90/10 principle by not letting negative external events affect us can significantly improve our lives and prevent unnecessary stress.