"The water boils."
Because in Italian articles undergo a phenomenon called "ellipsis" when they end in a vowel and the successive word starts with a vowel, to give it more fluency. e.g.: lo zucchero/l'animale; la pasta/l'acqua; una carota/un'ape. Only "uno" (indeterminate masculine article) is not elided but has the form "un" and does not respond to this rule: there's "uno scoglio" (=a reef) and "un animale" but also "un cane"; same applies to "il" (determinate masculine article together with "lo" and "l'") but I can't explain why since it comes natural to us :(
You're welcome. I figured since it was 7 months ago and you are now Level 13 with a 278 day streak, that you probably already knew the answer. But, you know how we answer good questions on a thread just in case new people would like to know the answer to the question. I also noticed, later on, that you, too, are now answering questions!