mkrlione, sorry, but nobody can say why because nobody knows what you exactly wrote. Women and men designed Duolingo courses but then, it's a machine who knows if your answer is rigth or not. No teacher here, only advenced learners who give explanations when they have the time.
Colin, "the coffee and water are hot" is approximative English. "The coffee" is definite and "water" is indefinite. It seems rather weird to me. Either both words are definite, nor both are indefinite. Think of your own language: would you say "the coffee and water...."? Not in French, my native language. In yours?