I cannot understand how entropy just happened
Let's have gas A and gas B. Gas A is hotter than gas B. The black line is a wall, and the gray grid represents the possible places the atoms of the gas can be in.
What do you suppose would happen if the wall was removed?
Entropy is the disorder in a system. What would need to happen for disorder to increase? The gases would have to mix. How about the other way around, what would need to happen for disorder to decrease? The hotter particle would have to move from the right side to left side, and the colder from left to right.
Now, let's remove that wall, let the system be for a while, then put the wall back and observe what has happened. Let's repeat this multiple times.
Here are the results:
..
pretend that here are more images with results like above
..
As I demonstrated, the disorder increased in all the scenarios except one. Entropy is a result of simple probability: there are much more possible outcomes for end results which are more disordered than originally, than those of less disorder. It is
possible that when we do this experiment we'll end up with a colder and a hotter gas, but the probability of that happening - due to there being much, much more disordered states than ordered states - is
highly improbable. It will happen
almost never.
As for the actual topic, read
a book.
Or two. They're both very well-written and can explain the matter much better than I can refer their content from my memory.