In the second part of the presentation you've learned that views are organized on the screen according to a layout. Layouts are views themselves, which allows them to be nested within another layout. It's possible to position views in relation to one another, provided that each view is given a unique identifier. So this allows for a nice graphical user interface, but it lacks interactivity. And what about interactivity? We need to do something about that. So we're going to work on that right away. The other side of the activity board is black, because the user will not see this side. But on this side, we're going to stick another set of magnets to control the white side, the displayed screen. There is quite a range of actions, that we can specify for when the OK button is clicked, for instance. And there are multiple events we can react to in addition to the click on the OK button. So, let's see some of these controls. First, I can change the appearance of the button. Like its background color, for instance. When the event 'button_OK is clicked' appears, then I'll do the following action: I'll set the background color of this view to the value 'blue'. So the button is clicked, and, it turns blue. Or I can make it invisible. The event is still the same, the OK button is clicked, but now the action is to change the visibility of the OK button. Knowing the ID of an editText can also allow me to retrieve the text typed in into it by the user. So for instance here, when this button is clicked, my first action is to retrieve the text stored in the view EditText_op1. That is this view. So here let's say that the user entered 3, I retrieve this value and I store it into the variable called nb1. Next my second action is to retrieve the text typed in the second EditText, EditText_op2. Let's say the user enters 4, so now the variable nb2 = 4, and my last action is to set the text displayed in this TextView here, TextView_result, to be the value nb1 + nb2. So here I'll display the String "7". So we already have a very simple functional calculator, but maybe you have something more ambitious in mind, something which would be more natural to achieve using multiple activities. Well Android enables you to define as many activities and screens as you like for an application. And just as for views, each activity or screen is given a unique identifier. But at a given time of the execution of your application, only one activity is fully active. And this activity presents a screen to the user, and the user cannot interact with any other activity. So of course, an action associated with this interface can be to change the activity. Like here, when the OK button is clicked, my action is to change activity to start a new activity, an activity named act_items. So the user clicks, and here I have my new activity. The screen associated with the new activity comes into the foreground, and the user no longer interacts with the first activity, which is stopped. If the user clicks the back button on the phone, the current activity is destroyed, and the screen of the previous activity, which was stopped, is brought to the foreground again. When you move from one activity to the next, you can pass some information along. For instance, in the first activity I retrieve the age of the user, and I start the second activity, named activity act_howTo. And I can provide the value of the variable 'age' to this new activity. And open starting, the other activity can retrieve this value, and put it into a variable and then use it afterward. For instance here you can display a customized message. You can design plenty of Android applications as sets of activities and associated graphical user interfaces. In the next series of readings you will discover additional widgets, actions and events. And then you will be able to imagine something fun or useful to make our user happy.