On a clock the three hands - the second, minute and hour hands - are on the same axis. How often in a 24 hour day will the second hand be parallel to either of the two other hands?
Investigate the positions of points which have particular x and y coordinates. What do you notice?
How does the position of the line affect the equation of the line? What can you say about the equations of parallel lines?
This interactivity allows you to sort letters of the alphabet into two groups according to different properties.