I have a few different sets of bed sheets and they are made of different substances.  A couple of them, I am guessing, are flannel.  I don't know it for a fact but the word flannel evokes a feeling of safe, soft comfort - and that's what those bedsheets are like.  Warm.  Nice.  Whether or not they actually are flannel, that's how I think of them: my flannel sheets.
But then I have these other sheets that are a more slippery substance.  Could be satin I guess?  I don't know.  But when I go to bed in them I feel like an oyster that is about to slide down the throat of a big sweaty fat guy who breathes loudly and makes disgusting eating noises.  He is going to consume me even though he is not hungry at all and I am going to pass through his digestive system and be turned into runny feces and then be shit out while he sits on a toilet grunting and reading a Robert Heinlein novel.
I prefer the flannel sheets.