diff --git a/artworld/drawable.js b/artworld/drawable.js
index bd50e4f..e280c92 100644
--- a/artworld/drawable.js
+++ b/artworld/drawable.js
@@ -12,6 +12,7 @@ export class Drawable {
     this._strokeWeight = parent ? parent._strokeWeight : null;
     this._z_index = parent ? parent._z_index : null;
     this._posVec = new Vector2(0, 0);
+    this._animations = [];
     if (this._parent) {
       this._parent.addChild(this);
     }
@@ -26,8 +27,16 @@ export class Drawable {
     throw new Error("draw() must be implemented on child class");
   }
 
-  // do nothing by default
-  update() {}
+  animate(setter, func, args) {
+    this._animations.push({ setter, func, args });
+    return this;
+  }
+
+  update(t) {
+    for (let anim of this._animations) {
+      this[anim.setter](anim.func(t), anim.args);
+    }
+  }
 
   // setters
 
diff --git a/artworld/math.js b/artworld/math.js
index 8ea4263..b3d8609 100644
--- a/artworld/math.js
+++ b/artworld/math.js
@@ -33,6 +33,10 @@ export class Vector2 {
     );
   }
 
+  get magnitude() {
+    return Math.sqrt(this.x * this.x + this.y * this.y);
+  }
+
   static random(x, y) {
     let theta = Random.radians();
     // if neither specified, use (1, 1)
diff --git a/artworld/shapes.js b/artworld/shapes.js
index e57dda4..96e8387 100644
--- a/artworld/shapes.js
+++ b/artworld/shapes.js
@@ -206,11 +206,11 @@ export class Polygon extends Drawable {
 
   point(vector, to_modify = null) {
     if (to_modify !== null) {
-      self._points[to_modify] = vector;
+      this._points[to_modify] = vector;
     } else {
-      self._points.push(point);
+      this._points.push(point);
     }
-    return self;
+    return this;
   }
   // TODO
   // contains(x, y) {
diff --git a/artworld/world.js b/artworld/world.js
index 912f57a..7e2d1e4 100644
--- a/artworld/world.js
+++ b/artworld/world.js
@@ -9,6 +9,7 @@ export class World {
     this.targetFrameRate = 60;
     this.stepSize = 1000 / this.targetFrameRate;
     this.lastTick = this.timer.time();
+    this.numTicks = 0;
   }
 
   bindCanvas(id) {
@@ -39,8 +40,9 @@ export class World {
   }
 
   tick() {
+    this.numTicks++;
     for (let d of this.drawables) {
-      d.update();
+      d.update(this.numTicks);
     }
   }
 
diff --git a/examples/clock.html b/examples/clock.html
new file mode 100644
index 0000000..78e7f02
--- /dev/null
+++ b/examples/clock.html
@@ -0,0 +1,51 @@
+<html>
+  <head> </head>
+  <body>
+    <canvas id="mainCanvas" width="500" height="500"></canvas>
+    <script type="module">
+      import {
+        artworld,
+        Color,
+        Pico8,
+        Group,
+        Rect,
+        Random,
+        Vector2,
+        Line,
+        Circle,
+        degToRad,
+      } from "../artworld/index.js";
+      artworld.bindCanvas("mainCanvas");
+      window.artworld = artworld;
+
+      function colorFunc(t) {
+        return [Pico8.RED, Pico8.ORANGE, Pico8.GREEN, Pico8.BLUE][
+          Math.floor(t / 60) % 4
+        ];
+      }
+
+      function sizeFuncFactory(minSize, factor) {
+        return (t) => Math.sin(t / 100) * factor + minSize;
+      }
+
+      function timeAngle(t) {
+        return (t / 6000) * Math.PI * 2;
+      }
+
+      let g = new Group().pos(new Vector2(250, 250));
+      new Circle(g)
+        .fill(Pico8.BLACK)
+        .z(1)
+        .animate("radius", sizeFuncFactory(200, 50));
+      new Circle(g)
+        .z(10)
+        .animate("fill", colorFunc)
+        .animate("radius", sizeFuncFactory(190, 50));
+      new Circle(g).radius(20).fill(Pico8.BLACK).z(50);
+
+      new Line(g).to(Vector2.polar(200, 0)).z(100).animate("angle", timeAngle);
+
+      artworld.loopStep();
+    </script>
+  </body>
+</html>
