Recurrence problem about planes

Question

Find the recurrence relation satisfied by Sn, where Sn is the number of regions into which three dimensional space is divided by n planes if every three of the planes meet in one point, but no four of the planes go through the same point. I figured out that the nth plane should go through every two from the set of n-1 planes, however, I don't get how many regions that adds. It's difficult to visualise this.