What geometry theorem is dynamically being illustrated in the applet below? [br](Feel free to move any of the white points anywhere you'd like at any time.)
For a complete description, see this [url=https://twitter.com/CutTheKnotMath/status/839535085967847424]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url] ([url=http://www.cut-the-knot.org/]cut-the-knot.org)[/url].