What reference am I missing?

Wow. I've just started working with VS 2008 and WF and I ran into the coolest thing. I tried to build my solution and got the following error:

Error 2 The type 'System.Workflow.Activities.ExternalDataEventArgs' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Workflow.Activities, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. C:\Documents and Settings\xyz\My Documents\Visual Studio 2008\Projects\p1\p1\p1PageFlowService.cs 13 62 p1PageFlowService

I've never had any version of Visual Studio tell me which reference was missing.

As much as I hate to sound like I've been drinking the Kool-aid... Good going Microsoft!