Also, there's no surprise that it's addictive, because you <i>didn't include a natural place to stop</i>. Every time you submit something, you're immediately given something new to do, no break inbetween! Of course people are going to have a hard time stopping.<p>Edit: Let me also add, it's not immediately obvious that there's a way to see what becomes of your chains if you don't keep going.<p>(Also there's too many people who will just totally ignore what they're supposed to do, but others have already said that and I have no idea what you can actually do about that.)<p>Edit: Also, the mismatch between the drawing box size and the display box size can be a problem if the boundaries of the picture matter.