By default, nothing. However, if you install the sl (Steam Locomotive) program, you can play a practical joke on those who enter sl by mistake.
First, install sl.
sudo apt-get install sl
In a terminal, enter sl and watch an animated ASCII steam locomotive chug across the terminal.
If that did not make your day, then try reading the humorous sl man page.
Now, let’s cause the passengers to scream for dear life by entering the accident option.
Look for the woeful cries of Help! as the pitiful passengers prepare to meet their impending doom.
Linux is too much fun!