That's not a problem. Closed systems can do that. If they additionally do not exchange energy with their environment they are called isolated systems. https://

That's not a problem. Closed systems can do that. If they additionally do not exchange energy with their environment they are called isolated systems. https://en.wikipedia.org/wiki/Closed_system https://en.wikipedia.org/wiki/Closed_system https://en.wikipedia.org/wiki/Isolated_system https://en.wikipedia.org/wiki/Isolated_system