Answer:
That's a great question -- and one that gets asked a lot.
Step-by-step explanation:
There are three explanations at work here, depending...:
1. There are many words of Greek origin that end in an a, but are truly masculine, for instance, el tema, el clima.
2. When a feminine word begins with a stressed 'a', "el" is used instead of "la." So:
"El agua" - (feminine)
"La avenida" - The avenue (feminine)
3. Some people say that the word would sound confusing or funny to the ear: 'lagua.'