Watermelon-ing: faking your way through a song you don't know the lyrics to by mouthing the word "watermelon" over and over.
This is mostly for those times when you are standing in front of people and it will be obvious that you are not singing along with everyone else. It works amazingly well!
Levi recently learned this little trick (we use it at church quite often) and was using it in full force as he sat in front of the whole congregation at church during a song he didn't know. We locked eyes and he knew he had been caught watermelon-ing his way through the song. It was hard to hold back the laughter, but we did our best - we were in the middle of church!
Next time you are supposed to be singing in a group try watermelon-ing your way through a verse or two. For practice sing watermelon to the tune of "Joyful Joyful We Adore Thee"