They will move via a crooked path because of something called the mean free path because there are air molecules present. And these air molecules will create collisions. And this green molecule, for example, will first collide with this guy, then move here, close the wall, then collide with this guy, and so on until it gets to some point here. Now, when the green guy reaches the red guy, something will happen. Well, the reaction is as follows. The green guy reacts with the red guy, or ammonia reacts with hydrochloric acid.