Posts do not stay on your main blog page forever. You decide when you set up your blog how long you want your posts to remain on the main page. Display setting might be: "display posts for 10 days," or "display the last 20 posts." So what happens to the post when it is 11 days old and it disappears, or when it is post number 21? It automatically goes into the archive of your blog.

Each post always has a permanent link to that individual post. So when it disappears, you can use that link to go to the post. Sometimes the permalink, as it is called, appears under the post and says: permalink. Sometimes the title of the post is also a permalink to that post.

So how do you get the permalink to a post that no longer appears on your main page? You can use your blog's search function to find the post. Locate the permalink on the post when you find it. And then you can paste that link in a new post to refer to the original post.

Here's an example:
"Remember class, we first learned about the order of operations three weeks ago. You can refresh your memory by clicking on this link to that post. Now we will learn an additional step to this process." In this example you would include the permalink to the original post, which has long since disappeared from your blog's main page, on the order of operations in the bold text.

Click on the permalink in the footer of this post to see what an individual entry link produces.