Create a new Sheet
Insert - Worksheet
Create the following spreadsheet
Click on cell D2. Type in =B2*C2 and press Enter
Click on cell D3. Type in =B3*C3 and press Enter
Click on cell D4. Type in =B4*C4 and press Enter
Click on cell D5. Type in =D2+D3+D4 and press Enter
Block cells B5:C5 and click on AutoSum button 
once.
Click on B5 and take note of =SUM(B2:B4)
C5 is =SUM(C2:C4)
Change the number of items e.g B2 to 5, and notice the changes
The cell references in this example are called Relative References.
