You can move the points A and B by clicking them and dragging them with your mouse. By doing this you'll see the two crossed lines rotating. Take a careful look at the size of the angles between those lines.
The sizes of the angles always follow an important rule (theorem). What rule? Look at the angles as you move the lines, and try to work out the rule. It's very simple.

When you think you know, press the "Show the Rule" button at the bottom of the screen and the rule will be revealed.
Enjoy yourself while you practise and learn this important rule by name.