Reactions forward and backward are still occurring. It's just they're occurring at the same rate. And that's exactly why the concentrations are remaining the same. Now suppose, for example, if I added more x, if I add more x, I would change my concentration of x. And that means for a few seconds or for some time, my concentration of x would change and this would shift equilibrium this way, causing x to be converted to Y at a faster rate than y converted to x. And that's called leisurely air principle and we'll talk about that in another lecture.