Rewrite Rules: Adding a trailing slash

I'm stuck on the code challenge about adding a trailing slash. The question is:

In an earlier code challenge, we wrote a rewrite rule so that the web page [http://localhost/flavors/] executed the code at [all_flavors.php]. Unfortunately, this only works if the web address includes the trailing slash. If someone visits [http://localhost/flavors], they will see a 404: Page Not Found error. We'll need another rewrite rule to redirect from [/flavors] to [/flavors/]. Add a new line to the htaccess file below to accomplish this.

I've tried: RewriteEngine On
RewriteRule ^flavors/$ /all_flavors.php RewriteRule ^flavors$ /flavors/

and various types of Regex that I think go beyond what was taught. Does the order of the lines matter? Why do we need to redirect to flavors/ if that will just redirect to all_flavors.php (I've tried doing it directly, too)?

Any hints as to what to try?