The term 'THEY TURN LITMUS PAPER RED' is referring to ACIDS, which are substances that have a pH level of less than 7. ACIDS are capable of turning litmus paper red due to their low pH level, which is why the answer to this crossword puzzle is ACIDS. ACIDS are found in many everyday items, such as vinegar, lemon juice, and even soda. They are also used in industrial processes, such as metal etching and food production.