Linking to cells in a separate spreadsheet

I'm not talking about a different sheet within the same spreadsheet. I'm talking about two different spreadsheets. I'm trying to reference cells from one spreadsheet that contains data I need to put into the current spreadsheet. It's easy in Excel. But I can't figure out how to do this at all in Numbers. It seems only to allow you to create linked references in sheets within the same spreadsheet. Please help!