Just found out: Clean inside of a thermos - Simple!
The throat of the bottle (Full-metal thermos) was too narrow for normal cleaning-brushes...
I fortunately had a bag of rice standing around and thought "Hm... Hard and small things"!
So i quickly added half a handfull of dry rice in the thermos and a bit of cold water (2-3 spoons).
Then i shook it a bit and poured the rice out.
The inside was quite clean and almost "pristine" again.
This method should also work for thermoscans with glass-interiors.
Another (better) way of doing that is propably (but ONLY if you have a METAL-Interior!) to add a hand of sand or fine gravel with some wather (Maybe even a bit of dishsoap) and give it a shake.
Again: Only do the sand/gravel if your thermos does not have a glass-interior!!
Have fun and enjoy a good drink while outside! :)