First three paragraphs of a short publication titled "Beginners’ Guide to Asset Allocation, Diversification, and Rebalancing":

"Even if you are new to investing, you may already know some of the most fundamental principles of sound investing. How did you learn them? Through ordinary, real-life experiences that have nothing to do with the stock market.

For example, have you ever noticed that street vendors often sell seemingly unrelated products - such as umbrellas and sunglasses? Initially, that may seem odd. After all, when would a person buy both items at the same time? Probably never - and that’s the point. Street vendors know that when it’s raining, it’s easier to sell umbrellas but harder to sell sunglasses. And when it’s sunny, the reverse is true. By selling both items - in other words, by diversifying the product line - the vendor can reduce the risk of losing money on any given day.

If that makes sense, you’ve got a great start on understanding asset allocation and diversification. This publication will cover those topics more fully and will also discuss the importance of rebalancing from time to time."

Beginners’ Guide to Asset Allocation, Diversification, and Rebalancing (SEC)