It’s just the steam, really. My grandfather used to play this trick on us all the time. He would steam a big stick of butter and bend it in front of us to pretend he was super strong. As the older siblings started to catch on, we would start protesting the trick and reveal his secrets, but then he would just shout over us “Margarine Mancy! Margarine Mancy!” And then stuff our pockets full of butter until we either laughed or cried. Of course this would ultimately be his undoing when he died at the race track.