What is mass?
We have always thought of mass as an intrinsic property of a material substance. The discovery that all material substances are made of some fundamental particles just shifts the problem of understanding the nature of mass from substance to its constitutions. However, recent developments in understanding the nature of particles and their interactions that have culminated in the standard model of particle physics have revolutionized our understanding of mass. Mass is not an intrinsic property of a material substance. It is not something that matter just has. It is a behavior of the quantum fields. It is something that they do rather than what they are. The objective of this talk is to explain to a general audience this new understanding of the nature of mass.