How the segment addition postulate can be modeled using a real-world object?.

Postulate: given segment AC the point B belongs to this segment if and only if  segment AB + segment BC = segment AC.

Then you can use a stick and call A and Cits edges, the mark an intermediate point, i.e. between A and C, on the segment and verify that you can measure each segment AB, BC and AC ant the postulate is satisfied.