water pipes are usually made from copper.sections of pipe are held together with solder,which can stretch?

(this is the continuation of the question) without cracking.PLEASE EXPLAIN WHY THE SOLDER IN THE PIPES IN A CENTRAL HEATING SYSTEM MUST BE ABLE TO STRETCH.

If the pipes expand from the heat, but the solder doesn't. guess what happens to the joint. If the solder expands and contracts with the pipe, it reduces the thermal stresses on the joint, and reduces the likelihood of leaks.

Copper expands and contracts with temperature change. Solder will also expand and contract but it does not fatigue.