The whole reason you’re clean is because you left most of your dirt and gunk in the bathwater. So the layer of water that keeps you wet after you get out of the bath, is a layer of *not so clean* water. When the towel soaks that up off of you, it absorbs the gunk dissolved in the water. And when the towel hang dries and the water evaporates, that gunk is left behind.
There’s also going to be some dead skin cells, and oils on your skin and hair, even after washing. which remain on your body and can rub off on a towel.
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