int x = 120;
int y = 60;
int radius = 12;

void setup() {
  size(240, 120);
  ellipseMode(RADIUS);
}

void draw() {
  background(204);
  float d = dist(mouseX, mouseY, x, y);
  if (d < radius) {
    radius++;
    fill(0);
  } else {
    fill(255);
  }
  ellipse(x, y, radius, radius);
}

void keyPressed() {
  radius = 12;
}