i have 2 files a.xls and b.xls

i wanna cross-reference data from a.xls to b.xls such that when i drag ,
values in cell A1 to A10(referenced from a.xls) will be filled
automatically in cell A1 to A10 of b.xls.

Currently, when i drag the cell A1, what it does is just duplicating A1
values all the way to cell A10.

How do i set it such that it will perform in the way i hope ?