sum of adjacent angles on a straight line
We can clearly see two lines. One is blue and the other one is black. We can change the position of these two lines with free points A, B, C and D. We can see that the two lines are intersecting and making two adjacent angles α and β. Let us move the free points A, B, C and D and observe how these two angles get changed.
Questions to think about 1. Move the points A, B, C and D and find α+β, what do we observe? 2. Is it possible to get α+β ≠180˚?