Suppose this green molecule wants to go from this corner to this corner. Now, the best way would be to go directly across from this point to this point. But notice that we have a bunch of red molecules in the way. So this guy will have to push its way to the other side. And by pushing, I mean colliding. So it's going to travel some distance A, then distance B, then CDEF, and finally G.
So each time it collides and it bounces off.