How do we know anything? And how can we know things better? Michael de Podesta explains why measurement is so important. Subscribe for regular science videos: http://bit.ly/RiSubscRibe Measurement is at the heart of all scientific endeavours. And underpinning every measurement is the International System of Units – ‘The SI’. In 2019 the world will change its definition of four key SI units, including the unit of mass (the kilogram) and temperature (the kelvin and the degree Celsius). Instead of defining these units in terms of arbitrary standards, we will switch to making measurements based on the natural constants of the world around us. Michael de Podesta's is a scientist at the UK’s National Physical Laboratory (NPL). His wide-ranging research interests concern all aspects of temperature measurement: from building the most accurate thermometer ever made; to developing industrial sensors capable of surviving harsh conditions; to measuring the temperature underneath the wheel of a train travelling at over one hundred miles per hour; to representing NPL on the steering committee of the International Surface Temperature Initiative. The Ri is on Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for the latest science videos: http://bit.ly/RiNewsletter