"The soup is healthy even though it contains salt."
Translation:Die Suppe ist gesund, obwohl sie Salz enthält.
Because "die Suppe" (the soup) is a feminine noun. That means you do not only use a feminine article ("die"), but also refer to the soup as "sie" (she) and talk about "her flavour", etc. In short, grammatically speaking you treat the soup as if it were a woman. The same goes for masculine and neuter nouns. By contrast, in English all inanimate objects are referred to as "it", so you can't translate the sentence literally into German.
When the American writer Mark Twain tried to learn German in the 19th century, he amused himself by imitating what this would sound like in English. You can read his story here:
Twain's use of the genders is surprisingly accurate, actually. :)
the rain = der Regen (masculine)
the hail = der Hagel (masculine)
the snow = der Schnee (masculine)
the mud = der Matsch/der Schlamm (masculine)
the fishwife = das Fischweib (neuter - note that the word "Weib" is considered derogatory in modern Standard German)
the scale = die Schuppe (feminine)
the mouth = der Mund (masculine)
the sound = der Ton/der Laut (masculine)
With most nouns, no, unfortunately - you have to learn the gender by heart when you learn the word. However, with some words you can tell the gender from the ending or other things. See here:
Gender hints masculine nouns: http://german.about.com/library/blgen_der.htm
Gender hints feminine nouns: http://german.about.com/library/blgen_die.htm
Gender hints neuter nouns: http://german.about.com/library/blgen_das.htm