Take a look at your worksheet. You can click on any cell to remove the black highlight. Do that now by clicking on Cell D3. Look up at the status window beneath the menu bar. Scroll through the other cells and you will see that they are all slightly different.
Spreadsheets make use of what are called relative cell addresses in copying formulae.
When a formula is copied, the program doesn't maintain the old cell addresses. It recalculates them relative to the destination. It may sound a bit confusing at first, but relative cell addresses are one of the many blessings of spreadsheets. We are nearly at the end of this first exercise. Once again we will need to lay the framework for out work.