The riddle refers to ginger (adrak). The clues match perfectly: 'come from under the ground' - ginger is a root; 'thousands of years, I've been around' - Daadi's letter states 'ginger has been used for cooking for more than 4000 years'; 'for cooking you use me daily' - ginger is extensively used in Indian cooking; 'from cold I can cure you surely' - Daadi mentions 'Adrak (ginger) is another herb that can help us when we have cough and cold.'