First, put the Oxiclean in the carafe. I used a partial scoop of Oxiclean, just above the line on the plastic scoop that comes with the product.

Fill the carafe halfway with hot water from the tap. Cover and shake.

Open and fill the rest of the way with hot water. Let it sit for a while. I let it sit for two hours; you can probably do it for less time.

The carafe is like new.

I’m passing this on because it does not seem to be widely known. Every time I Googled for how to clean a stained Thermos, I saw a lot of tips about denture tablets and dishwasher soap, which don’t work well at all in my experience. And the shape of the carafe makes scrubbing impractical.

Are whistles particularly clean, by the way?