"Why is the soup sour?"
Translation:Warum ist die Suppe sauer?
The usual answer is that "warum" and "wieso" are perfectly interchangeable, but in this case, I think "wieso" sounds a little bit more wondering or agitated ("Strange - potato soup shouldn't be sour, right?", "Wife! Why is that soup sour again?!"), and "warum" sounds a little bit more like you actually want to know how it came to pass ("I think it's because I added a glass of lemon juice, Mr Kitchenchef.").
Duolingo suggests "saür" because "ue" is an alternative spelling for "ü" for when you don't have an ü on your keyboard, but in this case Duolingo is wrong, since "sauer" is genuinely spelled (and pronounced) with "ue". Two seperate vowels, not an "ü".