Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/887294595452850176]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]The applet below contains a [b][color=#bf9000]concave quadrilateral[/color][/b]. (If you don't remember what the adjective [i][b][color=#0000ff]concave[/color][/b][/i] means with respect to polygons, [url=https://www.geogebra.org/m/knnPDMR3]click here[/url] for a refresher.) [br][br]You can move the [b]BIG VERTICES[/b] of this quadrilateral anywhere you'd like. [br][b][color=#ff00ff]You can also control the size of the interior angle with pink vertex using the pink slider. [/color][/b] [br][br][color=#1e84cc][b]How would you describe, in words, the phenomenon you see? [/b] [br][/color][color=#ff00ff][b]How can we formally prove what is informally illustrated here? [/b][/color]