I would start by explaining what problems they try to solve, like:
We need content to show -> HTML
This content needs to be styled and positioned -> CSS
This content needs to be interactive -> JS
I am sure you can come up with more cases, but I believe that taking a project oriented approach is the most effective when it comes to learning. (It also spares the thought of "why one would need it" that just simply shreds enthusiasm)
You can also quickly see what it is used for and what not. 'Breaking the rules' should only come later.
I would start by explaining what problems they try to solve, like:
I am sure you can come up with more cases, but I believe that taking a project oriented approach is the most effective when it comes to learning. (It also spares the thought of "why one would need it" that just simply shreds enthusiasm)
You can also quickly see what it is used for and what not. 'Breaking the rules' should only come later.