Move the tangrams to construct a square. Drag the interior to change their position and use the 'blue point' on the vertex to rotate.
You might want to see my [url=http://mathandmultimedia.com/geogebra/]50 GeoGebra Tutorials[/url] and my own [url=http://geogebracentral.blogspot.com/]collection of GeoGebra applets[/url].