Spreadsheet has an auto-calculation feature. This feature is turned on by default, so Spreadsheet automatically calculates the results of all formulas after each entry you make that affects a formula.
To turn off auto-calculation, select the Auto Calculate command from the Tools menu: the check next to the command in the menu will be removed. Select the Auto Calculate command again to turn it back on.
When auto-calculation is disabled, you need to select the Calculate Now command from the Tools menu to calculate the results of formulas. Manual calculation of this sort is useful if you have many formulas, or are putting in a lot of data, and you do not want to wait for Spreadsheet to recalculate after every entry that affects a formula.
Auto Calculate and Calculate Now operate on the current spreadsheet as a whole and not just on a selected cell or range.