For thousands of years, ghosts and spirits have fascinated many, who believe that they are the souls of people who have died and who have entered a realm beyond the physical.
I have a theory on what ghosts and spirits actually are.
I believe that spirits are nothing paranormal at all, but may in fact be terrestrial in origin. I think that they are actually a type of animal species that exists as pure energy (how this occurs I do not know) and are 'invisible' to us because we lack the ability to see them.
They are like radio waves. We cannot see radio waves but with equipment we can detect them, because they exist.
I believe spirits live in the upper parts of the atmosphere where they feed off solar radiation.
Going further into my theory, I might sound ridiculous but I also think that spirits are able to shape-shift.
This shape-shifting ability might come naturally. If spirits feed off the energy of a dying creature, it could aquire the creature's appearance and traits. Of course, as pure energy, spirits are able to transform themselves at will anytime they wish to.
We do know of parasitic creatures in Nature that feed off their hosts' bodies and acquire their physical appearance and traits, so spirits, as terrestrial beings, could have the same ability.