To see how GeoGebra works, let's build a figure. Assuming the vertices on a triangle are labeled A, B, and C, you can construct a triangle with a right angle at B by following these steps:
1. In the Geometry window, click on the point icon on the toolbar, and click on any two points in the Geometry window to create and display two points named A and B. {Be careful to not place points to far apart.}
2. Join the two points by clicking on the line-segment icon in the line drop-down on the toolbar and clicking on A followed by B.
3. Choose the perpendicular line icon on the toolbar. Click on the line segment AB and then on point B to draw a line perpendicular to AB at B.
4. Create circle with a center at B and radius BA, the intersection of the circle and the perpendicular line is named C.
5. Repeat steps 3 and 4 to find a perpendicular to BC at point C and create point D.
6. Right-click on the perpendicular line you created in step 3 and uncheck Show Object to hide the perpendicular line from view in the Geometry window.
7. Use the line segment tool and join point B to point C, then point C to point D and point D to point A.
Now, only the square ABCD is visible.

Are any of the point free to move? Which one(s)?
Describe (a paragraph or two) what happens by your actions of moving the free points.