Height
Magnus Bane and Alec Lightwood walked into Takis. When they got to the table were Jace,Clary, Isabelle and Simon were Simon said "Your so big." Alec blushed getting the unintended innuendo. Then Jace said "That's what HE said,"
A/N if you don't get it Simon means that Magnus is tall. But everyone else gets the innuendo about a different body part.