-- Take a washable marker, and give your little sister another washable marker.
-- Stand back-to-back in your bedroom, you facing the front wall, and her facing the back wall.
-- When you say 'GO', both of you draw a straight line on the wall. Then look at the lines.
-- The front and back walls are parallel planes, but it would be amazing if the two lines are parallel. One could even be a horizontal line and the other a vertical one . . . definitely not parallel.
