This is week number four of the course. In the introduction, we've mentioned that Scratch is a descendant of the famous Logo turtle program. Nowadays, turtle programming is still one of the best ways to teach programming to beginners. It can set up different instructions to draw different shapes using the turtle cursor. In the example, the small triangles have different length. It calls for the usage of variables. A variable is a container that we can use to store data. We will visit the same container over and over to check for new values. We can also imagine variables as the same person picking different colored dresses, and we use a variable to keep track the color of the dress. You can also imagine your weight as a variable. Sometimes it can be 45kg, sometimes it can be 60kg, and we use a variable to keep track the weight of a person. With variables, we can make conditional statements, like, when my weight is more than 70kg, I will drink less coke. After we master the use of variables, we can combine several conditions to deal with more complicated situations. For example, the situation when condition A and condition B both happen. To repeat tedious work for us is a primary design of computers and robots. We have been using indefinite repeats that stop at vague condition. Now, after we have learned how to use variables, we will have more controls over the repeats by defining strict conditions.