Babies are weaned on the stuff. School cafeterias stock it by the half-pint. You can order a glass at any local diner, donut shop, Burger King, or McDonald's, and you can buy it at the supermarket and the 7-Eleven. Coffee milk, a pale butterscotch-colored concoction of milk and coffee syrup, has been a Rhode Island staple since the 1940s. Six years ago, despite vociferous protest by the lemonade lobby, it was named the state drink. Rhode Island stores sell premixed coffee milk in cartons, but purists prefer to blend their own, using two tablespoons of coffee syrup for every 8 ounces of milk.