Most of us know what we mean when we say that something has happened. For theoretical physicists, however, this isn't an easy question. Sean Carroll of the California Institute of Technology explains why it's hard to define events and what to do about it.

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?.