One of the most important and fundamental concepts in linear algebra is the vector. Luckily, vectors are all around us, but they are, in general, not visible. The common ways to introduce a vector is either to begin with the strict mathematical definition, or to discuss examples of vectors, such as velocities, forces, acceleration, etc. For a more intuitive and hopefully faster understanding of this important concept, this chapter instead begins with an interactive demonstration and a clear visualization of what a vector can be. In this case, a ball’s velocity, which consists of a direction (where the ball is going) and a speed (how fast it is going there), is shown in Interactive Illustration 2.1.