ssh -X Error: Can't open display: :0.0

[Mathieu Malaterre <mathieu.malaterre@gmail.com>]

Hi there,

  I am getting confused. I cannot get xclock to run on a remote host
from my ssh session:

local $ ssh -X mpi0
remote $ xclock
Error: Can't open display: :0.0

But I can still do:

local $ ssh -Xf mpi0 xclock

Could someone please remind me what is the difference, and what am I
missing in the first scenario ?

Thanks,
-- 
Mathieu