Most of us know what we mean when we say that something has happened. Theoretical physicists, however, struggle with the concept of an event. Why?

Anthony Aguirre, of the University of California, Santa Cruz, explains.

This video is part of our Stuff happens: the physics of events package. You can read more about the reasons why, when you go down to the foundations of physics, our ordinary notion of events goes out of the window, in our article Is it happening?.