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Because of the inside of the flask. If you don't wash it out properly or if you do, the heat of the heats the inside of the flask and you probably drink some plastic with the tea! Just a thought!
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If you don't rinse the thermos out right away, bacteria can grow that are truly nasty and taste bad.
Also, if you are using dish soap, this can leave a bitter or fragrant residue if you don't wash it all off.
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