The document discusses the 90/10 principle developed by Stephen Covey, which states that 10% of life is made up of external events outside of one's control, while 90% is determined by how one reacts to those events. It provides an example of how reacting negatively to a minor incident like spilled coffee can ruin the whole day through a series of related events, whereas reacting positively can prevent that outcome. The principle teaches that one cannot control what happens but can control how they respond, and choosing to respond well positions them to have a better experience overall. Mastering this skill allows one to avoid unnecessary stress and problems.