Answer: Segment Addition Postulate
This is the idea where we can take any line segment and break it into smaller pieces, then glue those pieces back together to get the original line segment.
The original line segment is PR. Point Q is somewhere between the endpoints. The smaller pieces are PQ and QR. Adding PQ with QR forms PR again. In the real world, you can do this with a stick or string, though this assumes there are no wasted bits that are thrown away or lost.
