iPod (with Dock Connector) models come with a Dock Connector cover plug.
You insert the cover plug into iPod’s Dock Connector port on the bottom of iPod. This plug protects the connector, and keeps foreign matter out of the opening.
To put iPod in its dock, or to connect the Dock Connector to FireWire cable, remove the plug. iPod will not fit into its dock when the plug is installed.
If your old iPod is missing its Dock Connector Cover Plug, you can get on one eBay for $8.