Brush your teeth, or else!
So maybe it’s a little extreme, and my Waldorf-educating friend may not approve, but my kids were giving me a horrible time about brushing their teeth, so I threathened them with the following image of an enemy Strogg from the latest Quake game:
The desired response was achieved, with two gleefully squealing kids yelling ‘EWWWW’ and running of to brush their teeth. For the record, the kids do not not play Quake, nor will they any time soon. And, those teeth aren’t any worse than Grimm’s Fairy Tales – and just as effective!

