Liquid not working [newbie]

Hello all,
I am going through the tutorial. I get stuck at step 2. Li quid.
Liquid notation without frontmatter is, predictably, displayed as-is. If I add frontmatter, my index.html file is ignored and the default “Welcome to Jekyll!” screen is displayed.
This is my index.html file:

# front matter tells Jekyll to process Liquid
<!doctype html>
    <meta charset="utf-8">
    <h1>{{ "Hello World!" | downcase }}</h1>

My mistake must be something terribly simple, but I have no idea what it is.

do you have both an and an index.html? you can only have one or the other.

I have both, yes. I renamed to, now Jekyll picks up my index.html file, and Liquid is processed correctly.
Thank you!