Wordpress uses index.php instead of single.php for posts

I am creating a Worpdress theme and want to show my posts. I created single.php, but when I open the message, it displays it in index.php. Since single.php is higher in hirarchy than index.php, this should not be.

For single.php, I tried a very simple version, for example

<?php get_header(); ?>
Showing single.php
<?php get_footer(); ?>

and, of course, I did not have erros. I also tried copying single.php from twenty-twelve. In both versions I have the same problem.

Can anyone help? Thank!