Ubuntu: Switch to a user

Hi,

1) I have a linux workstation (Ubuntu 10.04.3)

2) I open a terminal and it appers like this:

Jack@ubuntu-10:~$

3) Now, I want to change to "root"; i do the followings:
sudo -s
4) And it changes like this --> root@ubuntu-10:~#

5) After quite sometimes, I want to change back to "jack@ubuntu-10:~$"
- I issue the command --> sudo jack
- The message: sudo: jack: command not found

6) My question: What is the command to change from "root@ubuntu-10:~#" to "jack@ubuntu-10:~$?

7) Thank you

tjie