derivative with respect to time. But what does it even mean to take the derivative of a probability density? That's a hard thing to wrap your head around. Fortunately, physics has been dealing with this problem for centuries. It's the fundamental phenomenon of transport. This involves the movement of a conserved quantity, such as electric charge flowing through a wire or water flowing through a pipe. Probability mass behaves similarly. It is also a conserved quantity as it always integrates to one. It doesn't spontaneously appear or disappear. Instead, it simply moves through time and space. This type of transport is described by a classical result from the 19th century, the continuity equation. The continuity equation gives us an expression for the partial derivative that we didn't know how to tackle earlier. It involves a time-variant velocity field VT that transports the density over time. But what exactly is a time-variant velocity field?