SQL Server contains self-tuning functionality that will recompile your query as the source data changes to find more efficient query plans for your current data.  This process of recompiling queries is a great, general-purpose feature to keep your system running well.  We have a white paper describing some best practices for managing query recompilation in your server that is a good primer for learning more about how your system is working "under the covers".  Enjoy!

http://www.microsoft.com/technet/prodtechnol/sql/2005/recomp.mspx

Thanks,

Conor Cunningham