float angle = 0;
int inr = 0;
void setup()
{
size(500,500);  
background(120,120,120);
}
void draw()
{

if (int(angle) >= 360)
{
angle = 0;
inr = 0;
}
background(120,120,120);
translate(width/2, height/2);
angle++;
rotate(radians(angle));
inr = inr + 1;
rect(-25-inr, -25-inr, 50+inr, 50+inr);
}