Angles and Crossed Lines
This applet demonstrates a simple rule about crossed lines and vertically opposite angles.
Author and programmer: Ron BarrowTweet
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.
When you think you know, press the "Show the Rule" button at the bottom of
the screen and the rule will be revealed.