It is a beverage that normally appears brown and is normally shown in a glass cup.
Role in series
"Tea at the Treedome"
No tea is actually drank in this episode until the very end, where Sandy puts the teabags in SpongeBob and Patrick's new water helmets, which turns the water into tea. Prior to this, Sandy invites SpongeBob over to her treedome for tea and cookies.
Squidward drinks tea at the beginning of this episode and spills it when he's shocked at the sight of Squilliam appearing on the eponymous television series. He later refers to it as the "hideous stain on the rug."