Full of gas!

I asked the students to tell me the meaning of this English idiom: Empty vessels (Containers) make the most noise.  The students always seem to read idioms too literally even when you tell them not to; and the students did do that yet again this afternoon.  None could tell me what the idiom meant in a general sense.  However, one student inadvertently said something profound: she said that the vessels made the noise because they were full of gas.  Who, pray tell, has been making empty gaseous speeches the last year?