There was a time I had a "seance" when I was maybe 12 or so, with two of my friends in my room. We were sitting in a circle, holding hands, with our eyes closed (or, at least mine were, so I guess I can't confirm theirs were too) and one of my friends was saying some mumbo jumbo trying to bring the spirit of her dead grandfather to us. We had no idea what we were doing and were basically just winging it for fun.

The way I remember experiencing it: My eyes were closed and I was focusing. I started to hear a tapping that sounded strangely to me like a helicopter outside the window. I remember thinking "what's that weird sound?" and then I opened my eyes and saw my desk chair, a few feet away from us, moving. It's hard to explain, but the back of the chair moved in a way that allowed it to hit the spine of the chair, and it looked like (I only saw for a split second) the back of the chair was moving up and down and hitting the chair spine, causing the tapping noise.

Since I was so young I don't remember when my friends opened their eyes or what, I just remember looking at the chair for a second and all three of us screaming and running out of the room. My parents, although religious (liberally), basically told us we were crazy and to go to bed.

Nothing else like this ever happened to me, and to this day I'm convinced that either one of my friends was playing a trick (although if they were, they were excellent tricksters for 10 and 12 years old) or that our minds were playing tricks on us.

Since I have never heard of or seen a story of spirits interacting with humans that has had any scientific backing to it, I accept that this was very likely not a supernatural experience and more likely a psychological one.