Farmers' markets are terrific, of course, but in St. Louis the peak of the market season coincides with temperatures near 100 degrees and humidity that makes it feel like you're living inside someone's mouth. Maybe that's why the cries of "Lemonade!" that resound from Sunflower Savannah's stands at the Tower Grove and Maplewood farmers' markets are as enticing as a siren's song. This is the Platonic ideal of lemonade: lemon juice, water and just the right amount of sugar. Neither too tart nor too sweet, after a morning or late afternoon spent lugging around bags filled with potatoes, tomatoes, eggs and meat, it's the perfect thirst quencher.