I have a flashlight which I use when I have to get up in the night in the pitch-black room. It takes three AAA batteries.
Recently, it has been rather dim, so I assumed that I would have to switch batteries soon. First, I took the current batteries out, put in brand new ones, and tested it. Much brighter, as expected.
Then I took out the new ones again and put the old ones back in, to compare. Now it was almost as bright as with the new ones again.
This is not the first time in my life that I've had similar things happen. A remote controller for a television set might seem dead, but once you open the battery hatch and "roll around" the batteries a bit, or take them out and put them back in again, it again works.
How is this possible? How can this (apparently) cause new energy to flow into the batteries? Surely this cannot be what is happening?