On May 20, the definitions for the units of mass, charge, temperature and mole will change. This is a major change in the international system of units, eliminating all man-made objects from these definitions. This talk will provide some historical background and motivation for defining fundamental base units. However, the main focus of the talk is: How do we, as scientists or engineers, explain these units to the general public? For the redefinition of charge, temperature and mole, this is rather straightforward, but even physicist struggle to explain how fixing the numerical value of Planck’s constant h provides the new definition of the kg. Conceptually, the explanation is that 1 kg is now the mass of a defined number of photons, 1.4755214*10^40, at the frequency of the cesium atomic clock. However, this gets us into questions like: Does a photon have mass? Is a mole of carbon still exactly 12 g? How accurately can you count a huge number of photons or atoms, and finally, how do you realize this conceptual definition in practice (by using the Watt or Kibble balance, or a single-crystal silicon sphere)?