[b]Step 1.  Experiment One[/b][br]Use the MOVE [icon]/images/ggb/toolbar/mode_move.png[/icon]tool to move the Drag Point up and down.[br]What happens to the distances between the blue and purple lines as you do this?[br]Move the Drag Point so that the red distance line is a little bit less than 40 units long.[br]Select the ZOOM OUT tool[icon]https://tube.geogebra.org/images/ggb/toolbar/mode_zoomout.png[/icon].[br]Click on the Zoom Here point several times.  [br][br]What is going to happen to the blue and purple lines if you zoom out far enough?[br][br]Click on the UNDO BUTTON to return to the original display.[br][br][b]Step 2.  Experiment Two[/b][br]Move the Drag Point so that the red distance line and the green distance line are both 40 [br]units long.[br][br]Now select the ZOOM OUT tool [icon]/images/ggb/toolbar/mode_zoomout.png[/icon] and again click at the Zoom Point several times.[br][br]What can you say about the blue and purple lines now?[br][br]If the distances between two lines at any two points are not equal, the lines will eventually meet and cross each other.  They are [b][i][color=#ff0000]intersecting lines[/color][/i][/b].[br][br]If the distances between two straight lines at any two points are the same, the lines will never intersect.  They are said to be parallel lines.[br][br]Railroad tracks are a good example of [b][i][color=#ff0000]parallel lines[/color][/i][/b].  As you look way down the tracks, it may appear that they intersect at some point, but they never will. How can you be sure?  When the rails are attached to the railroad ties, the track builders are very careful to measure and make sure that the two rails are always exactly the same distance apart.