diff --git a/docs/sheets/2-animation.md b/docs/sheets/2-animation.md
index fbf2a5a..a6f9a66 100644
--- a/docs/sheets/2-animation.md
+++ b/docs/sheets/2-animation.md
@@ -323,18 +323,7 @@ Finally, if we don't destroy the bullets, eventually the game will get really sl
     ```
     _This should go inside the bullet's `forever`, just after the code to move it._
 
-The `destroy()` function attached to a Sprite removes it from the screen permanently.
-
-However, we haven't quite cleaned up after ourselves--the forever block for the bullet will keep running, even though it's no longer on screen! This isn't super-important, but in larger games it might cause us to run out of memory. So let's `return false` to stop the `forever` block from running:
-
-  * We should also `return false`, to make sure the `forever` block stops.
-
-    ```js
-        if (!bullet.isOnScreen()) {
-            bullet.destroy()
-            return false // stop this forever block
-        }
-    ```
+The `destroy()` function attached to a Sprite removes it from the screen permanently. This also stops any `forever` loops attached to it.
 
 
 ## Fin
diff --git a/uw/index.js b/uw/index.js
index c1c0851..1593286 100644
--- a/uw/index.js
+++ b/uw/index.js
@@ -153,7 +153,6 @@ var World = function(props) {
     document.body.style.margin = '0px'
     document.body.appendChild(this._wrap)
     this._resize()
-    this._deadLoops = []
 
     window.addEventListener('resize', () => { this._needsResize = true })
     this._bindPointer()
@@ -174,7 +173,7 @@ var World = function(props) {
     this.stop = this.stop.bind(this)
     this.start()
 }
-emitter(World.prototype, ['frame', 'tap', 'drag', 'drop'])
+emitter(World.prototype, ['tick', 'tap', 'drag', 'drop'])
 
 World.prototype.start = function() {
     if (this.isRunning) return
@@ -194,7 +193,9 @@ World.prototype.pause = function() {
 World.prototype.stop = function() {
     if (!this.isRunning) return
     this.pause()
-    this._deadLoops = this._deadLoops.concat(this.listeners('frame'))
+    for (var i=0; i<sprites.length; i++) {
+        sprites[i].destroy()
+    }
 
     window.removeEventListener('blur', this.pause)
     window.removeEventListener('focus', this.start)
@@ -250,15 +251,24 @@ World.prototype._frame = function() {
 }
 
 World.prototype._tick = function() {
-    // remove stopped `forever` loops
-    for (var i=this._deadLoops.length; i--; ) {
-        this.unlisten('frame', this._deadLoops[i])
-    }
-    this._deadLoops = []
-
     // trigger `forever` loops
-    this.emitFrame()
+    this.tick()
+    const sprites = this.sprites
+    for (var i=0; i<sprites.length; i++) {
+        sprites[i].tick()
+    }
 }
+World.prototype.tick = function(cb) {
+    // TODO opt?
+    var dead = []
+    for (let fn of this.listeners('tick')) {
+        if (fn() === false) {
+            dead.push(fn)
+        }
+    }
+    for (let fn of dead) this.unlisten('tick', fn)
+}
+World.prototype.forever = World.prototype.onTick
 
 World.prototype._draw = function() {
     if (this._needsResize) this._resize()
@@ -558,7 +568,7 @@ const Base = function(props, init) {
     this.dead = false
     world.sprites.push(this)
 }
-emitter(Base.prototype, ['tap', 'drag', 'drop'])
+emitter(Base.prototype, ['tick', 'tap', 'drag', 'drop'])
 
 // TODO: cache rotated sprites.
 
@@ -798,6 +808,8 @@ Base.prototype.getTouchingFast = function() {
     return result
 }
 
+Base.prototype.tick = World.prototype.tick
+Base.prototype.forever = Base.prototype.onTick
 
 
 /* Sprite */
@@ -1119,18 +1131,6 @@ Sound.prototype.play = function() {
 
 
 
-// forever
-
-function forever(cb) {
-    if (typeof cb !== 'function') throw new Error('oops')
-    const w = world
-    w.onFrame(function listener() {
-        if (cb() === false) {
-            w._deadLoops.push(listener)
-        }
-    })
-}
-
 
 /* events */
 
@@ -1185,7 +1185,6 @@ module.exports = {
     Polygon,
     Rect,
     Sound,
-    forever,
 }
 Object.assign(module.exports, maths)