During Prohibition people found ways to sneak around the law. Winemakers, facing ruin because of the law, found an ingenious way to sell the product of their vineyards. Instead of making the wine on their premises, they created "wine bricks" out of concentrated grape juice for home brewers (and bootleggers) to dissolve and use in the privacy of their own homes.
The bricks were marked with warnings that they were for non-alcoholic consumption only. The “warning” helped people make wine at home. According to VinePair’s Adam Teeter:
If you were to purchase one of these bricks, on the package would be a note explaining how to dissolve the concentrate in a gallon of water. Then right below it, the note would continue with a warning instructing you not to leave that jug in the cool cupboard for 21 days, or it would turn into wine.Link