The noLoop() function causes draw() to only execute once. Without calling noLoop(), the code inside draw() is run continually.
y = 180 def setup(): """ The statements in the setup() function execute once when the program begins """ size(640, 360) # Size should be the first statement stroke(255) # Set line drawing color to white noLoop() def draw(): """ The statements in draw() are executed until the program is stopped. Each statement is executed in sequence and after the last line is read, the first line is executed again. """ global y background(0) # Set the background to black y = y - 1 if (y < 0): y = height line(0, y, width, y)