I should add that copper, lke other metals, can be flexed or bent many times as long as you stay within its elastic limit. Each time you exceed that, you do a little more damage until ultimately it breaks. Extension cords and flexible power cords use relatively fine wire and thick insulation on the wire and the jacket which tend to resist sharp bends. Stranded wire is usually laid in 7, 19, 37, 51 or 81 strand configurations This is done because it makes for a compact and stable geometry based on a hexagon. Most low cost extension cord are usually 19 strand. Flexible wire used for electronic or automotive purposes is typically 7 strand as is flexibility is intended for ease of installation and once installed, it moves very little, if ever. Cables used in automation have to be super flexible and generally have a high strand count..
Fun fact: Farms out in the plains had telephone service before Chicago did. The farmers and ranchers used their barbed wire fences for connecting the phones with a short cable running from the fence to the house to complete the circuit. Two strand wire, of course. Wooden fence posts were used in stead of steel posts so insulators weren't necessary. They may have had telephone outages during heavy rain storms though.
