I used to love the "Words of one song to the tune of another" in the wonderful panel game "I'm sorry I haven't a clue" on BBC Radio 4. We had this in action one morning in our school assembly when the music teacher was bashing out one hymn while the words to something which did not even vaguely fit were projected on the screen for the "congregation". Pupils made a valiant attempt until the music teacher realised in about verse 3. A classic moment in the education of those young minds!