From the Archives of CTC, Inc:
To start a new line within a spreadsheet cell, use one of these options:
1. In Microsoft Excel, press Alt + Enter to create a new line within a cell. You can create many new lines in a cell by repeatedly pressing the Alt + Enter key combination.
2. In OpenOffice, LibreOffice Calc and Google Sheets, press Ctrl + Enter to create a new line within a cell. As with Excel, you can create many new lines in a cell by repeatedly pressing the Ctrl + Enter key combination.
