I'm watching this clip Friends: Someone Ate Ross' Sandwich (Season 5 Clip) | TBS. At 0:53, Ross hands over a food note to Chandler and Chandler starts to read it (transcript quoted from here):
Ross: ... I still can't believe someone ate it!! I mean, look, I left a note and everything.
(Shows the note to Chandler who reads it aloud.)
Chandler: (reading) Knock-knock. Who's there? Ross Geller's lunch. Ross Geller's lunch, who? Ross Geller's lunch, please don't take me. Okay?
Joey: I'm surprised you didn't go home wearing your lunch.
After Joey says "I'm surprised you didn't go home wearing your lunch", the entire audience laughs. But I don't understand why Joey's line is funny. Could anyone help me?