The teacher shows an animation that presents one of the most common situations in which a linear function is involved. When we make a phone call from a phone booth or from a cell phone, we think about how many minutes the phone call lasts. Why? Because there is a relation between the minutes we have talked and the cost of the phone call. In one of the phone companies in our country, one minute is $425. If you talk for two minutes, you will pay $850, and so forth. In order to determine the behavior of the cost of any number of minutes, you can establish a function that relates the two variables: minutes spoken and cost of the phone call. Here you will learn about important aspects about the linear function and some of its most common applications.