 In this video, I'm going to draw a set of right angle bookends. Each bookend will have a 50 pixel wide base and a height of 100 pixels, and they're going to be spaced 40 pixels apart. And here are the comments that say exactly that at the beginning of my program. Before I start programming, I'm going to draw a picture of what it's going to look like when it grows up. This doesn't have to be a fancy picture as you can see here. I just want to get an idea of what it's going to look like. And most important, I want to add numbers to it so I can see how much space I'm going to need everywhere. The red dot there in the middle, by the way, is where the turtle begins moving. My parents always used to tell me, do I have to draw you a diagram? And my answer in this case is yes, you do have to draw me a diagram because it really helps me figure out what's going on. So having this diagram in mind, I can now start the program. The first things we already know, I have to import turtle. I have to set up a window by calling the screen method, and I'm going to have a turtle T1, which is a turtle object. Let's draw the right bookend first. Looking at the diagram, the easiest thing to do will be to move 70 pixels east, and then I can back up 50, turn left, which will get me facing north, and go up 100 pixels. The question is, how do I move 70 pixels east without drawing anything? And the answer is to use the pen up method, which says bring the pen up off of the screen when you move. Then I can move forward 70 dots, put the pen down again, move back 70 dots, and that will, excuse me, back 50 dots, and that will draw my 50-dot wide base. I can then go left 90 degrees and go forward 100. Rather than try and write the entire program before testing it, let's see if that part works. So I'm going to run that, and sure enough, I'm getting the bookend that I want. It's a bit thin, so I'm going to add something here. Let's add T1 dot pen size 3 to make a thicker line and run again, and that's looking good. Now it's time to draw the left-hand bookend. I really don't want to try and move the turtle back to the beginning point or back to the far end of the left-hand bookend. So what I'm going to do instead is I'm going to create a second turtle, and it's going to start at the home position, right in the middle of the screen facing east. I can make it raise its pen, and this time I'm going to go back 70 pixels, because I'm going to the left, I'm going west. Put the pen down. This time I'm going to go forward 50 dots. I'm still going to need to go left 90 degrees, and then draw the vertical of the left-hand bookend. Let's save that and run it, and I forgot my pen size on the second turtle, so let's put that in, run it again, and now I have my bookends exactly as I wanted them. Did I really need to use two turtles here? No, not really. It turns out that there is a method for turtles called home. If you say home, that returns the turtle to its original place in the middle-facing east without having to do a lot of work. So instead of creating that second turtle, I could have done t1.home, and then gotten by just fine with one turtle. Let's show that, and oopsie, that doesn't work, because I forgot to bring the pen up before I went home. This is the sort of thing that happens all the time. If you start writing without planning, you get weird results like that. Not a problem. We'll add a pen up before we return to the home position. And in fact, because the pen is up, I don't need this. I didn't delete the line. The reason I didn't delete the line is because, just in case I'm wrong and I do need it again, I don't want to have to retype it. So instead I did what's called commenting it out. If everything works, I can delete the line later, but normally I will comment things out. So just in case I make another mistake, I can go back without having to do a lot of extra work. So let's think of what the computer is going to be doing. I've drawn my right hand bookend. I take the pen up off of the screen, and then I return to the original place, and then I go back 70 pixels, which is exactly what I want to do, because I still want the pen to be off of the screen. Doing the pen up again wouldn't hurt, but it wouldn't help either. This time it ought to work, and I've gotten by with one turtle. The question is, should you always use only one turtle, or should you use multiple turtles? Ask me if I care. Do you care? No, I don't really care a lot. I happen to be a one turtle kind of guy, and I like to do things with one turtle if at all possible. But there are some algorithms, some drawings that you want to do, where it will be a lot easier and a lot more convenient to use multiple turtles. It's somewhat of a balancing act. If using multiple turtles makes things harder to read, then go back to fewer turtles. It's a judgment call. But I will not be disappointed if you do the assignments and get them done with one turtle or multiple turtles, as long as the program is done and working properly. The second thing we can take away from this video is that sometimes you will write a program and it won't come out the way you want. Don't panic, don't freak out. Analyze it, figure out when went wrong, and correct it, and that's the world of programming. Welcome to the Our World.