The external, as well as the internal are forever changing, no matter what we do, say, or try to actively control on a daily basis. Nothing is static, everything is in constant motion, fluctuating, growing, evolving, dying; turning into something that it was not just months, weeks, days, seconds, or even moments before. This constant change, that nothing lasts forever, is the law of impermanence, which is defined as the state or fact of lasting for only a limited period of time.
Even if you’ve never heard of this concept, I’m sure the word “impermanence” seems somewhat familiar. If you take a closer look at it, you should see the root word permanent, which comes from the Latin permanere, "endure, continue, or stay to the end." This means that nothing every changes, it always stays the same, no matter what. Think of a permanent marker. If you write on a wall or someone’s clothing, there’s no way of washing it off; it’s there forever, you can’t change it or erase it. On the other hand, you have impermanence, which is the exact opposite of permanent. It means nothing lasts forever, nothing stays exactly the same, and to understand the impermanent nature of everything in life is to understand the inevitability of change.