How to add excerpts to posts and pages
To add an excerpt to a post, simply write one in the Excerpt field under the post edit box. An excerpt can be as short or as long as you wish. Usually, given its purpose, a couple of sentences is fine.
Note: As of WordPress version 3.1, some screen options on the Post & Page edit Administration Panels are hidden by default. The Excerpt field is hidden by default if it has not been used before.
Pages doesn't have excerpt by default. To add an excerpt for your pages similar to the one for your posts you'll need to add this code to the functions.php:
// Add excerpts to Pages. add_post_type_support( 'page', 'excerpt' );