forgetting lyrics

I'm on the computer last night with Grease playing on the TV via TCM. I'm singing along with the movie just fine until the very last song which I can never remember all the words to. When it gets to the words I don't know I sing "la la" in tune with the melody.

What do you do when you're singing a song and can't remember the lyrics?

Stop singing all together
Hum or whistle along
Sing along using generic syllables like "la la"
Sing the lyrics you think might be right but probably aren't
  • Current Music: Garbage - "Sleep Together [Twisted Mix]"
I would have added that option but the do-what-I-want and damn-the-consequences notion sounded too much like what George W. Bush would do.