What??? A forgotten memory ???

Why is it that when one begins to wake-up...just coming out of sleep...a long forgotten memory surfaces?  How is that possible?  How 'do' our minds work?  I wasn't even awake, but my mind showed me a mental picture of another dear ol' dad memory.
So I'm just beginning to wake-up, as I just said, and I see, in my mind's eye, this sheet that is covering a wing-backed chair, with steam coming up and out through it, in the living room of my parents' house.  And as I come to myself, I think,  "I remember that!"
Yeah.  My alcoholic dad was such a kidder.  Made me a little afraid of him at times, though.  He liked to play pranks on people...mostly his naive children or irritated wife.  Yeah.   You'd probably refer to those as being punked nowadays.   He liked to do the oddest things to surprise us, and to laugh at us, and generally get a really good kick-out-of-it...for his own pleasure. 
He was a witty man, very witty.  Almost like Archie Bunker, when it came to mom, and like the bully on the Little Rascals when it came to us.  But, I can say he kept our life interesting.   And he could tell stories that made your hair stand-up on your skin...all over your body! 
But, I was remembering that chair with that sheet and that steam, wasn't I?  So, my sister had fallen asleep in 'his' chair.  And, since she hadn't gone-off to bed like a good little girl, and had inconvenienced him, he decided to teach her a lesson and have some fun at the same time.
He took the old vaporizer, and a sheet and pulled it over the wing-back chair.  Then, we all sat back and watched as the steam engulfed that chair, and sheet, and my sister.  I really don't remember what "scent or odor" that dad put on that cotton ball, in the little cup that was used for mentholatum to easy a child's breathing.  But, knowing dad it must have been bordering on cruel.
So, there we sat.  And one thing dad hadn't counted on was that, my sister couldn't be awakened for anything, once she had fallen sleep.  I knew  that for sure.  Nights that I was afraid of 'whatever'...I'd try to wake her --- to no avail.   And so, we all just sat there waiting for her to get spooked, I guess, but the only thing she did was to move her arm and pull that sheet down, foiling dad's little scheme.
Funny, I had thought I remembered all there was to remember, when writing in my journals, and eventually in my books.  But here I am this morning, awakened to a memory that I had totally forgotten.  Just goes to show  ya' --- there's more in one's memory bank than initially meets the mind's eye...see