Liquid not working [newbie]

Help
1

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>
<html>
  <head>
    <meta charset="utf-8">
    <title>Home</title>
  </head>
  <body>
    <h1>{{ "Hello World!" | downcase }}</h1>
  </body>
</html>

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

2

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

3

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