Auto Increment gone crazy:
Sometimes Excel«s "cleverness" can get in the way.  For instance, you have entered "1999" into a cell.  You now want this identical number in four or five adjacent cells.  If you drag on the fill handle, Excel tries to be clever, and guesses that you wish to enter "2000", "2001" and so on.  Darnn!  Now you have to go back and edit the contents of these cells.  This can be annoying.

To get round this, enter "1999" into TWO cells, select both cells, and THEN drag on the fill handle.  Excel tries to auto increment the number, but increments them by zero.  Voila!  a neat little row of "1999"«s.

Stay!  Sit!  What«s going on here?
Here«s the scenario:  You«re back in lesson 5 (The VLOOKUP exercise), and instead of entering the VLOOKUP formula every time in cells B13, C13, D13, and E13 - you get a brilliant idea.  Work out the formula for cell B13, and then just drag on the fill handle to get Excel to autoll the other three cells.  You try it, and - huh?  It doesn«t work.  What«s gone wrong?

What has happened is that Excel tries to be too clever for its own good.  It reasons like this:

The VLOOKUP formula for cell B13 is as follows:
=VLOOKUP(B6,E17:F20,2)

Now, if you drag the fill handle into cell C13, Excel says: Aha!  Since we«ve moved from cell B13 to C13, I«ll be helpful, and automatically change "E17:F20" to "F17:G20" - Oops - NOT!  Every VLOOKUP formula in this example has to look at precisely the same lookup table.  So how to force Excel to always look at the same cells even when other parts of the formula are changing?  Simple.  Just add a dollar sign in front of every part of the cell address you do NOT want to change.  Like this:

=VLOOKUP(B6,$E$17:$F$20,2)
If you dtrag THIS formula, the parts of it with a dollar sign will remain the same - ie it will always look at the range of cells E17:F20

This is called ABSOLUTE referencing.