cannot run X apps when 'su'

[ttv <thanhvunguyen@hotpop.com>]

Hi, when I su as root, I can't run X apps ...


Camranh:/home/thanh# Xlib: connection to ":0.0" refused by server
Xlib: Client is not authorized to connect to Server
emacs: Cannot connect to X server :0.0.
Check the DISPLAY environment variable or use `-d'.
Also use the `xhost' program to verify that it is set to permit
connections from your machine.


any ideas how to fix it ? 



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