When one does not know the exact words to a specific song, said person can simply mouth the word "watermelon" in tune with the flow of the song to avoid looking like a complete and total asshole.
Rosalba- this song literally just came out like a minute ago how do u know the lyrics already?

Chris: psh girl I'm doin the watermelon trick

Rosalba : ahhhh shit
by The Sherman July 14, 2011