The sawing in half illusion using transparent boxes that James Corden performed on Elle Fanning is called Clearly Impossible. The performance came about after Elle revealed during her previous appearance on the show that she's a big fan of magic and would love to have a go at being sawed in half by a magician, but has never had the chance despite always volunteering at all of the magic shows she goes to see. So James promised that, the next time she was on the show, he would saw her in half.