"Who is eating the fruit?"
Translation:Hvem spiser frukten?
No, a noun can't just change its gender :) 'frukt' is a strictly masculine noun, so the masculine form must always be used.
Feminine nouns are an exception, they can be inflected as either a masculine or a feminine noun, but you'll have to be consistent (you cannot change its gender in the middle of a text).
Then there are oddballs, nouns that can be all three genders, and some nouns that change meaning once you change its gender ('et plan' (a plane) vs 'en plan' (a plan)), so it's important to use genders correctly.