Half modeling

How can I model only have of something and have the other have looking exactly like it?
I saw a video on you tube when a guy was sculpting a face, he only did half of the face and the other half was happening by itself.

use the mirror modifier
search the wili page for it
SAlutations

Yes, as RickyBlender stated you can add a Mirror modifier to your Base mesh before you sculpt. However, you can also, use X, Y, or Z Symmetry in Sculpt mode. Just click on the Sculpt tab and choose one or more axis of Symmetry you want to use.