Get a root prompt without assigning a root password

In other GNU/Linux distributions you can switch to a root user prompt with su. However, in Ubuntu no password is set for the root user, and sudo is used instead. You can still get a root prompt in Ubuntu by typing:

$sudo su

OR

$sudo -s

source:
http://kubuntuguide.org/Feisty#Get_a_root_prompt_without_assigning_a_root_password