Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/823536283918147584]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url] ([url=http://www.cut-the-knot.org/]cut-the-knot.org)[/url]. [br][br][b]Note: [/b][br][color=#0000ff]This applet does not work perfectly under all circumstances. [br]It works best if the quadrilateral (to be formed) remains convex and if point [i]K[/i] lies outside the quadrilateral.[/color] [br][br][i][color=#ff00ff]How can you formally prove what this applet informally illustrates? [/color][/i]