var posYE = 100; 

var raio = 25; 

function setup() {

  createCanvas(300, 300);

}

​

function draw() {

  background(220);

  if ( mouseIsPressed && dist( posXE, posYE, mouseX, mouseY) < raio) {

    posXE = mouseX;

    posYE = mouseY; 

  }

​

  ellipse(posXE, posYE, 2*raio, 2*raio); 

}