1. From the File Menu Select New, Spreadsheet. What happened? You got a new empty spreadsheet
2. Put a 1 in cells B2 and B3
3. In cell B5 put in formula to total cell B2 and B3
4. Change cell B2 to 100. What happened? The total changed because the spreadsheet recalculated the formula
5. On the Tools menu click on Cell Contents. Notice the tick against AutoCalculate and the shortcut keyboard for Recalculate is F9
6. Click on AutoCalculates so the tick is removed. You can verify this by clicking on Tools menu click on Cell Contents again. Leave it un-ticked.
7. Change cell B2 to 300. What happened? Cell B6 did not change. This is because Recalculation is switched off
8. Press the F9 function key (The function keys are the one above the normal keys). What happened? The spreadsheet recalculated because pressing F9 is the same as clicking on the Tools menu, Cell Contents, recalculate.
9. Use what you learn to switch auto recalculation back on and make sure that it is back on.

Auto recalculation is usually only switched off when you have a very large spreadsheet such that it is taking a long time to recalculate and you want to change a large number of cells and their is a substantial delay after changing each cell. Having it off removes that delay but you need to remember to recalculate the sheet or switch the recalculation back on to insure you have the correct results.

NOTE: If you save a spreadsheet with auto recalculation switched off it will still be off when you open that file

You have leant how to switch off and on automatic recalculation, how to force recalculation and that the recalculation status is saved when you save a spreadsheet.