Strategy: You can easily add an informative tooltip to any cell. The tooltip will appear when someone selects the cell.
1) Move the cell pointer to the cell. From the menu, choose Data – Validation. On the Validation dialog, go to the Input Message tab, as shown in Fig. 1493.

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Note: In Excel 2007 and later versions, the Data Validation feature is found on the Data tab.
2) On the Input Message tab, type a title for the tooltip. In the message area, type instructions for the person filling out the worksheet, as shown in Fig. 1494.

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Result: When you move the cell pointer to that cell, an informative tooltip will appear, as shown in Fig. 1495.

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Summary: Using the Input Message of the Validation dialog allows you to create helpful tooltips, which can appear in any cell.
Commands Discussed: Data – Validation
Related Features: Instead of an Excel tooltip, you may want to use comments instead. See Microsoft Excel: Leave Helpful Notes with Cell Comments for more details.