As you work with a file, everything you do is stored in only one place: random access memory or RAM. The contents of RAM are a lot like the light in a lightbulb—as soon as you turn it off or pull the plug, it's gone. Your hard disk provides a much more permanent type of storage area. You use the Save command to copy the workbook file in RAM to disk.
It's a very good idea to save documents frequently as you work. This ensures that the most recent versions are always saved to disk in the event of a computer problem.
Choose File > Save or File > Save As (Figure 47), press , or click ...