Copper strands when flexed multiple times or pulled on or stepped on will weaken by getting narrower and perhaps even breaking. So an extension cord may look fine on the outside, but internal wire damage causes the resistance in these weakened areas to rise causing increased temperatures there, which causes deterioration in the wire's insulation such as hardening and cracking. The situation can get bad enough to cause arcing and then the real possibility of a fire. Too often an extension cord is continued to be used even when a casual inspection of it should lead one to realize it should be discarded.
The NEC has extensive specifications on the spacing of receptacles in a kitchen and other normal living areas of a house so that an extension cord should not be necessary in order for an electrical device to reach a receptacle. This is based on the history that extension cords have been directly responsible for many fires, etc. Unfortunately they are just too inexpensive and easy to use. Only when dual GFIC and CAFI circuit breakers come into widespread use (only starting to become available), will there be fairly good means to identify wiring problems which will fortunately also include extension cords.
