Translation:When ice melts, it becomes liquid.
I guess I'm glad you checked that. My romantic illusion is shattered. I visualized young people melting with emotion over their inamorata/o and drooping into a boneless puddle on the nearest sofa. Now you tell me it is just foolish :)
The good news is that the "fondue" may be as foolish as I suspected.
For some reason I understand these as fount. It's officially a back formation from fountain, and so very distantly related indeed, but fits too well with all of those senses. Not "fond of" though -- that's just sick! That's subtle in English. For "fou de ~" I'd use the more literal English idiom "a fool for ~" -- well, IMHO . . .
Except that "elle devient" is translated as "it becomes". We can understand it in our minds that the H2O changes from a solid state into a liquid one and think of this as "turning into" something but that doesn't change the verb "devenir" into "to turn". We're not talking about English grammar here, or whether one prefers a different word, or how many times our preferred word can be Googled. Rather we need to focus on what the French words mean and learn to translate them faithfully while minimizing our own artistic license in the process. Having said that, if you prefer "it turns liquid", go right ahead. We know what you mean.
Quand la glace fond, elle deviant liquide. In this particular sentence deviant is wrong. In a previous sentence that is what Duolingo had is spelled as when it had me translate it into English. On this sentence Duolingo is stating that devient is the correct spelling. So which is actually the correct spelling deviant or devient please let me know.
Dictionaries and other resources are great aids to check up on spelling and conjugations. Here's a great one: http://www.conjugation-fr.com/conjugate.php?verb=devenir