Interactive geometry to use now

Pappus' theorem Three points, 1,2,3 and 1',2',3', are marked in green on each of two lines and then joined as shown. What can you say about the three points where the joins meet? If you can't see it, ask for a hint. Press "r" to reset the sketch. This problem is taken from "You Are A Mathematician" by David Wells (Penguin,1995)