
I could keep each item in its own cell and merge row headers. I could also keep each item in its own cell and repeat row headers. But what I really want to do is place multiple strands in a single cell, with each strand having its own line. Like this:

There is a simple keyboard shortcut that performs this task:
- Select the cell.
- Place your cursor in the formula bar where you will insert the hard return.
- Simultaneously press the Alt and Enter keys.