Pouring boiling water through fabric takes grape-juice stains right out. But you don’t get your cup of tea. You DO get to finish the juice-box.
I know. Club soda neutralizes grape-juice stains. There’s no club soda here. We don’t live in a bar.
Sippy-cups neutralize grape-juice stains but you have to apply them early.
To get permanent grape-juice stains out of cups fill them 3/4 of the way with water and the last 1/4 bleach. Soak for thirty-minutes or so before washing. If you forget and take a sip, spit like your life depended on it, until your mouth is desiccant.
