Excel's Data Validation feature allows you to restrict inputs within a cell. For instance, to limit inputs between 0 and 100, select a cell, then choose Data, then Validation. On the Settings tab, allow Decimal, then specify 0 for the Minimum and 100 for the Maximum. Fill in the Input Message and Error Alert tabs if you want to customize the prompts for that cell. Entering anything other than a number between 0 and 100 will result in an error prompt.