opening macro when the cell value is changed

HI,

Does anyone knows how to open macro when the cell value is changed in another workbook?

I have two workbooks and the cell value of FIRST workbook changes when the cell value of SECOND workbook is changed.
The requirement is to run a macro in FIRST workbook when a new value is entered in SECOND workbook. Thus the cell value should be the value from FIRST workbook.

Thanks