I saw daddy kissing Santa Claus
Underneath the mistletoe
Last night
He didn't see me creep
Down the stairs to have a peek
[Mike screams "Oh GOD!"]
He thought I was up in the bedroom fast asleep

Then, I saw mommy kissing Santa Claus
Underneath the Christmas tree last night
There must be some mistake
Was I really awake?
I move in closer, but better look away

[Mike and Billie make barfing noises]
[ Lyrics from: http://www.lyricsfreak.com/g/green+day/i+saw+my+parents+kissing+santa+claws_20374823.html ]