Presenter notes: Hold up a pressure cooker, or draw one, and point at the handle. Ask why the pot is metal and the handle is not, and let somebody say it would be too hot to hold. Then push: so the handle is made of a material that is bad at carrying heat, and we chose it for exactly that reason. Say the sentence that frames the lesson: today is about the elements that fail every test a metal passes, and about the fact that failing is a job. Then promise the two specifics, because both are close to home. By the end they will know why cutting an onion makes them cry, and why the salt packet in their kitchen has the word iodised printed on it.