Hands shoot up as he asks for a volunteer to work with the teacher, Chris Stutt, to devise a way of reaching a chocolate bar, placed under the distant basin. While 13-year-old Vinnie and Mr Stutt struggle against the clock the class bursts with advice.
When the chocolate is recovered, Barnes asks the class what went into the successful joint effort. Pupils suggest "sharing ideas, co-ordination, co-operation, negotiation" and other learning essentials. Many of these spontaneous offerings correspond with Claxton's four Rs - resilience, resourcefulness, reflectiveness and reciprocity - and their sub-headings such as planning, perseverance and collaboration.
Then Barnes asks the pupils what they do when they get stuck in lessons.
Ask the teacher, they reply. That, Barnes argues, is the wrong thing to do.
From now on "getting stuck" in lessons is not a sign of failure. In fact, it is desirable because when pupils start to tackle problems they learn how to solve not the one they face but a mountain of new problems further down the line.