Note to others: "The only reason that 'el agua' is correct and not 'la agua' is because in the second, there are two stressed 'a' sounds side by side. Hence 'el.'
This happens in all words, masculine or feminine, that begin with a stressed a, such as 'el águila,' meaning 'the eagle,' which coincidentally is a feminine noun as well."
(I am not complaining about duo.). When I am doing the exercises as fast as i can. I just hate this one question, because I'm keep typing "El pato bebe agua." and the answer they want is "El pato bebe el agua.." I don't even here the "el" until I click the listen to slower speech button. at least it is training my ear to listen. :-)