Blogging tip - new topic = new post
Always start a new topic with a new post.
Don't use comments to introduce a new topic. Comments should only be used to comment on the issue introduced in the post you are commenting on.
If you use comments to introduce a new topic you could end up with a long, rambling, confused thread.
Also, since only the post is visible when others read the blog, they won't become aware you have started a new topic unless they happen to delve into the post your comment is under.
If you have two or more topics you want to raise, make a new post for each topic. That way the comments for each will clearly relate back to each post.