Most people think plugging something in is a simple mechanical act. You push one side into the other, current flows, and the job is done.

In the real world, that tiny moment is a lot more complicated. Every connection depends on pressure, friction, surface chemistry, and the quality of two metal surfaces meeting at microscopic contact points. What looks smooth to the human eye is rough like a mountain range under magnification, and electricity only passes through the high spots where those surfaces actually touch.

That is where contact resistance begins. The less clean and stable those contact points are, the more resistance builds at the interface. Most of the time the change is small enough to go unnoticed. Over time, however, wear, oxidation, dirt, and repeated insertions can slowly turn a reliable connection into an inconsistent one.