track changes

I have an individual who needed to pass a spreadsheet to me with her changes. We initially thought she could just change the cell color, but instead she turned on the track changes so it would show what it had been changed from. She and made all appropriate adjustments but when she saved the spreadsheet and re-opened it the next day all of the "tracking" was gone.

What is the purpose of tracking changes if they are not saved?? Did we miss something that we need to turn back on again?? Is there a way to use the track changes and the tracking remains after the spreadsheet is saved and closed??