how to get tool tip for a column header

hi all,

i have a requirement that on 1 field of the alv grid

if user put cursor on the field header some text should be shown.

i searched for the field TOOLTIP in structure lvc_s_fact

but how to use it because its of type char and length 40 ,

what will i pass to pther fields for whom i dont want to show any tooltip.

please reply soon.