In real life, when an object at rest begins to move, it starts slowly. Similarly, when an object changes its speed, or direction, it rarely does so instantaneously, but rather makes the change gradually (that’s one reason we use splines to describe motion paths in computer animation). In order to create satisfying animation, it is important to be sensitive to the rate at which objects change their direction and speed; these factors are the most expressive component of path animation, like tempo and dynamics in music. In particular, the term slow-in/slow-out refers to an object at rest which gradually accelerates, reaches a final velocity, then slows and stops.