How do I change “read more…” to expand on the spot?

Hi Everybody,

You know how you can put a “read more” into your post, so that when someone’s on the front page, you don’t have to show the entire post, but if they click on “read more” it’ll take them to the entire post (the single page)…

How could I change that so that the “read more” link expands the post right there, so that they’re still in the main index and able to read the entire post?

Thanks