First Law of Thermodynamics

Work is expressed in such a way that a change in pressure is entirely internal and U doesn't change. On the other hand, a change in volume (dV) requires work to be done on the surrounding environment.

Thank you to be the first one for trying to explain. However, I still don't understand why pressure can't change in from an external force. Why must it be entirely from an internal force? Suppose that there's a random mass of gas in space and there's entirely nothing around. (Okay, that's not entirely true because there's always gravity, but let's just assume that the gravity is negligible or 0.) Then there can only be outward expansion because the molecules that are moving must collide into each other and bounce off of each other and travel towards nothingness.

Then the volume increases and the pressure decreases, and where's the work done? Also, if dW is already work done, what does integrating it and getting W give you?

I don't know anything about deriving equations or about the first law of thermodynamics, so excuse me if this post is stupid; however, it looks like people are trying to legitimately help you. They are giving you links of stuff that, to them, explains the problem just fine. Why would they link you to stuff they don't think will help you? What, is everyone out to troll you?

I wrote in the first post that I already saw the Wikipedia article. Linking me to it again without any other comments doesn't help at all. Doing this is clear cut trolling, because it assumes that the answer is completely obvious and common sense, and it implies that I'm moronic for not seeing it. If a child tells you that he is unable to learn from Sesame Street why things don't disappear when they're hidden, telling him to go watch it over again and again isn't going to help him.

But if the answer is so obvious, then it should be easy to explain too. Then why isn't anyone willing to do it? Because they're out to troll me. If the article explains the problem(s) just fine, then do you completely understand what the article is talking about? If you do, can you explain it to me in a way that will make me understand it?