Triangle Illusion

This set of 4 tiles can be arranged within the black rectangle in two ways.
To see these arrangements drag the red dot to the left and right along the red line.

Why does the second arrangement have a blank square? Click Here to see the answer. Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and activated. (click here to install Java now)

Created with GeoGebra