Probably the most familiar subject in physics is mass. Basically, it’s the amount of stuff something is made of. However, if you look at it a little more closely, you’ll find that the situation isn’t necessarily so simple. In this video, Fermilab’s Dr. Don Lincoln spends some time explaining how, conceptually speaking at least, there are two kinds of mass: gravitational and inertial and how the relationship between the two has huge consequences on our understanding of the universe.