Random cycling letters

Just wondering if anyone knows how its done.
For example the word “Welcome” From left to right random letters cycle through till the get the right one which reveals the word “Welcome”.

example attach.

site examples:
[COLOR=#003366]http://www.mocafusion.com/[/COLOR]
[COLOR=#003366]http://www.tronicstudio.com/media/about/about.swf (click mission btn)[/COLOR]