Download and install the Steam client on your PC. Including the epic single-player campaigns of both Metro 2033 and Metro.
After selecting New Game, you may choose from three difficulty settings: Easy, Normal or Hardcore. New Game Start a new story from the very beginning. When you click on the box an ajax request notifies the server that the div was clicked and the server stores this information in a temporary storage (marks the token: this token was activated by a human). Press any key at the Title Screen to be taken to the Main Menu. The checkmark is just another div (css class). Spambots are trying to fill the form input elements, but there is no input in the captcha. Please note that there is no checkbox at all, it is just a div element with some CSS styling. Since most spambots do not execute javascript and can not identify the correlation between the displayed text and the DOM or required actions they can not click on the checkbox.