"the_excerpt" in loop just keeps repeating first post?

Question

I'm trying to edit the loop in my archive.php template so that when I visit a category page, it only displays the post excerpt and uses the featured image as a thumbnail.

When I add the_excerpt() to the loop it displays the excerpt of the first post, but then just keeps repeating it indefinitely?

Here's the loop I'm trying to edit / modify:

// Start the Loop
while (have_posts())  {                 
    the_excerpt();
    get_template_part('content', get_post_format());
}

// Display navigation to next/previous pages when applicable
if (theme_get_option('theme_bottom_posts_navigation')) {
    theme_page_navigation();
}

Answer 1

the_post - Iterate the post index in The Loop. Retrieves the next post, sets up the post, sets the 'in the loop' property to true.

 while (have_posts())  { the_post();

after you trigger the_post your the_excerpt shold return different values.