函数允许你绘制图形,设置色彩,计算数据及其它各种功能。函数名字通常是小写后面加上一对括号,The comma-separated elements between the parentheses are called
parameters, and they affect the way the function works. Some functions have no
parameters and others have many.以下程序将介绍size()函数和background()函数。