Scratch This Boolean expression allows you to say to scratch if you are touching the mouse pointer, the little arrow, do something. How do we do something?

布尔表达式允许你告诉,当你触碰到鼠标指针，也就是小箭头，程序就相应地做一些事情？

哈佛公开课 - 计算机科学课程节选