Wait, what are the words again?

In my opinion there is nothing wrong with quietly singing along to a song that you may like. I do it myself and it helps to alleviate some of the stress I may be feeling at the time. The problem I do have is when I introduce someone to a song, and they try to sing along to it. First of all, they don't know the song, how the hell are they supposed to sing along if they don't know the words? Second, I am showing them the song for a reason, often because of the complexity of the lyrics or the possible meaning behind them. You can't listen to the lyrics if you're mumbling over them with words that aren't even there. So please, my one simple request: don't sing along if you don't know what you're singing.