diff --git a/artworld/drawable.js b/artworld/drawable.js
index 3ab659f..bd50e4f 100644
--- a/artworld/drawable.js
+++ b/artworld/drawable.js
@@ -26,6 +26,9 @@ export class Drawable {
     throw new Error("draw() must be implemented on child class");
   }
 
+  // do nothing by default
+  update() {}
+
   // setters
 
   fill(color) {
diff --git a/artworld/timer.js b/artworld/timer.js
new file mode 100644
index 0000000..09a49fe
--- /dev/null
+++ b/artworld/timer.js
@@ -0,0 +1,34 @@
+export class Timer {
+  constructor() {
+    this._pausedAt = null;
+    this._elapsed = 0;
+    this._lastStarted = performance.now();
+  }
+
+  pause() {
+    if (!this.isPaused) {
+      this._pausedAt = performance.now();
+      // store time since last pause
+      this._elapsed += this._pausedAt - this._lastStarted;
+    }
+  }
+
+  unpause() {
+    if (this.isPaused) {
+      this.lastStarted = performance.now();
+      this._pausedAt = null;
+    }
+  }
+
+  get isPaused() {
+    return this._pausedAt !== null;
+  }
+
+  time() {
+    if (this.isPaused) {
+      return this._elapsed;
+    } else {
+      return this._elapsed + (performance.now() - this._lastStarted);
+    }
+  }
+}
diff --git a/artworld/world.js b/artworld/world.js
index 1b7323b..24eefff 100644
--- a/artworld/world.js
+++ b/artworld/world.js
@@ -1,7 +1,14 @@
+import { Timer } from "./timer.js";
+
 export class World {
   constructor() {
-    this.drawables = [];
     this.ctx = null;
+    this.drawables = [];
+    this.backgroundColor = "white";
+    this.timer = new Timer();
+    this.targetFrameRate = 60;
+    this.stepSize = 1000 / this.targetFrameRate;
+    this.lastTick = this.timer.time();
   }
 
   bindCanvas(id) {
@@ -16,16 +23,39 @@ export class World {
     return this.ctx.canvas.height;
   }
 
+  // TODO rename
   register(drawable) {
     this.drawables.push(drawable);
   }
 
   draw() {
+    this.ctx.fillStyle = "white";
+    this.ctx.beginPath();
+    this.ctx.fillRect(0, 0, this.width, this.height);
+    this.ctx.fill();
     for (let d of this.drawables.sort((a, b) => a._z_index - b._z_index)) {
       d.draw();
     }
   }
 
+  tick() {
+    for (let d of this.drawables) {
+      d.update();
+    }
+  }
+
+  loopStep() {
+    window.requestAnimationFrame(() => this.loopStep());
+
+    let curTime = this.timer.time();
+    // tick appropriate number of times
+    while (curTime - this.lastTick > this.stepSize) {
+      this.lastTick += this.stepSize;
+      this.tick();
+    }
+    this.draw();
+  }
+
   // Canvas 2D /////////////////
 
   prepareDraw(drawable) {
diff --git a/examples/balls.html b/examples/balls.html
new file mode 100644
index 0000000..4f85eea
--- /dev/null
+++ b/examples/balls.html
@@ -0,0 +1,63 @@
+<html>
+  <head> </head>
+  <body>
+    <canvas id="mainCanvas" width="500" height="500"></canvas>
+    <script type="module">
+      import {
+        artworld,
+        Color,
+        Pico8,
+        Group,
+        Circle,
+        Random,
+        Vector2,
+        degToRad,
+      } from "../artworld/index.js";
+      artworld.bindCanvas("mainCanvas");
+      window.artworld = artworld;
+
+      class Ball extends Circle {
+        constructor() {
+          super();
+          this.speed = new Vector2(0, Random.between(9, 15));
+        }
+
+        update() {
+          this.move(this.speed);
+          if (this.worldPos.y > artworld.height + 20) {
+            this.y(-20);
+          }
+        }
+      }
+
+      class GravityBall extends Circle {
+        constructor() {
+          super();
+          this.accel = new Vector2(0, 0.5);
+          this.speed = Vector2.random(0, 5);
+        }
+
+        update() {
+          this.speed = this.speed.add(this.accel);
+          this.move(this.speed);
+          if (this.worldPos.y > artworld.height - 10) {
+            this.speed = this.speed.scale(-0.98); // bounce w/ dampening
+            this.y(artworld.height - 10.01);
+          }
+        }
+      }
+
+      for (let i = 0; i < 21; i++) {
+        new Ball()
+          .pos(new Vector2(40 * i, 0))
+          .radius(10)
+          .fill(Pico8.BLUE);
+        new GravityBall()
+          .pos(new Vector2(20 + 40 * i, 0))
+          .radius(10)
+          .fill(Pico8.PURPLE);
+      }
+      artworld.loopStep();
+    </script>
+  </body>
+</html>
