Season 5: Episode 6 - The Rhinitis Revelation

The Rhinitis Revelation: Sheldon's mother, Mary, comes to visit and the entire gang loves spending time with her, much to Sheldon's chagrin. He refuses to believe it's jealousy but, when he comes down with the common cold like a "common man", he's gleeful that mommy is there to take care of him and sing him "Soft Kitty."