Yesterday we discussed pressure differentials. Today we will be discussing relativity. Relativity tends to get lumped into discussions about the speed of light, Lorentz transformations, time dilation and other fun things. But really, relativity just means that what you observe happening depends upon your frame of reference.
For example, when you are diving in a small plane, the objects in the plane with you will begin to fall freely toward the earth. From the frame of reference of the ground, they are plummeting earthward. But from the frame of reference of the plane, you suddenly have a flying dog.