Theorem of three perpendiculars

Topic:
Geometry
Let P be a point on the plane . Draw the line r, perpendicular to plane in P, then a new line t in the same plane. Now draw the line s, perpendicular to t through P. Finally draw the plane , through r and s. Then the plane is perpendicular to line t. Move the points in the construction and see how the position of planes and perpendicular lines modify accordingly.