// ins feld klicken!!
int size = 30;
void setup()
{
  size(400,400);
background(0);
stroke(0);
}

void draw ()
{
if(mousePressed) { 
stroke(255);
}
else {
  stroke(0);
}
line(mouseX, mouseY, 0, 0);
line(mouseX, mouseY, 400, 400);
line(mouseX, mouseY, 0, 400);
line(mouseX, mouseY, 400, 0);
}