What did you say?

A conversation with Matthew today:
Matthew: Mom, what letters are in lion
Mom: L I O N
Matthew: What letters are in Matthew
Mom: M A T T H E W
Matthew: What letters are in FU
Mom: What did you say?
Matthew: FU
(I am now torn between wanting to tell him that is a terrible thing to say and to never, ever say it again and not wanting him to know it is a bad thing to say)
Mom: Where did you here that?
Matthew: It is a car
Mom: It is not a car
Matthew: Yes it is, it is what dad drives, an FUV
Mom: oh, (thank goodness) you mean a SUV