"What makes things sharp? Why are thinner knives sharper? What happens on the molecular level when you cut something?" All these questions came from Joshua Schwartz in New York City.

The ability to create sharp tools allowed us to fashion clothing, make shelters and hunt for food, all essential for the development of human civilisation, according to materials scientist Mark Miodownik.

We hear from IBM scientist Chris Lutz, who has used one of the sharpest blades in the world to slice up individual atoms.

Plus palaeoarchaeologist Becky Wragg Sykes reveals the sharpest natural object in the world, a volcanic glass used by the Aztecs called obsidian.

Presenters: Hannah Fry, Adam Rutherford