How to print excerpt and content in the head?

I want to create a simple plugin that prints the content and the excerpt in the head section. Below is the code I tried but didn't worked:

function content_excerpt() {
  if( is_single() ) {
  the_content();
  the_excerpt();
  }
}
add_action( 'wp_head', 'content_excerpt' );

via Chebli Mohamed