diff --git a/examples/invaders/main.rs b/examples/invaders/main.rs
index dd506b4..f4859d7 100644
--- a/examples/invaders/main.rs
+++ b/examples/invaders/main.rs
@@ -1,5 +1,7 @@
+use std::time::Instant;
+
 use pixels::{Error, Pixels, SurfaceTexture};
-use simple_invaders::{World, SCREEN_HEIGHT, SCREEN_WIDTH};
+use simple_invaders::{Controls, Direction, World, SCREEN_HEIGHT, SCREEN_WIDTH};
 use winit::event;
 use winit::event_loop::{ControlFlow, EventLoop};
 
@@ -26,6 +28,7 @@ fn main() -> Result<(), Error> {
     let surface_texture = SurfaceTexture::new(width, height, &surface);
     let mut fb = Pixels::new(224, 256, surface_texture)?;
     let mut invaders = World::new();
+    let mut last = Instant::now();
 
     event_loop.run(move |event, _, control_flow| match event {
         event::Event::WindowEvent { event, .. } => match event {
@@ -43,7 +46,18 @@ fn main() -> Result<(), Error> {
             _ => (),
         },
         event::Event::EventsCleared => {
-            invaders.update();
+            // Get a new delta time.
+            let now = Instant::now();
+            let dt = now.duration_since(last);
+            last = now;
+
+            // TODO: Keyboard and controller input.
+            let controls = Controls {
+                direction: Direction::Still,
+                fire: false,
+            };
+
+            invaders.update(dt, controls);
             window.request_redraw();
         }
         _ => (),
diff --git a/simple-invaders/src/controls.rs b/simple-invaders/src/controls.rs
new file mode 100644
index 0000000..e08268f
--- /dev/null
+++ b/simple-invaders/src/controls.rs
@@ -0,0 +1,17 @@
+/// Player control inputs.
+pub struct Controls {
+    /// Move the player.
+    pub direction: Direction,
+    /// Shoot the cannon.
+    pub fire: bool,
+}
+
+/// The player can only move left or right, but can also be stationary.
+pub enum Direction {
+    /// Do not move the player.
+    Still,
+    /// Move to the left.
+    Left,
+    /// Move to the right.
+    Right,
+}
diff --git a/simple-invaders/src/lib.rs b/simple-invaders/src/lib.rs
index 4382ed1..4879585 100644
--- a/simple-invaders/src/lib.rs
+++ b/simple-invaders/src/lib.rs
@@ -1,9 +1,19 @@
-mod loader;
-mod sprites;
+//! A simple Space Invaders clone to demonstrate `pixels`.
+//!
+//! This doesn't use anything fancy like a game engine, so you may not want to build a game like
+//! this in practice. That said, the game is fully functional, and it should not be too difficult
+//! to understand the code.
 
+use std::time::Duration;
+
+pub use controls::{Controls, Direction};
 use loader::{load_assets, Assets};
 use sprites::{blit, Sprite, SpriteRef, Sprites};
 
+mod controls;
+mod loader;
+mod sprites;
+
 /// The screen width is constant (units are in pixels)
 pub const SCREEN_WIDTH: usize = 224;
 /// The screen height is constant (units are in pixels)
@@ -25,6 +35,7 @@ pub struct World {
     score: u32,
     assets: Assets,
     screen: Vec<u8>,
+    timing: Duration,
 }
 
 /// A tiny position vector
@@ -132,11 +143,47 @@ impl World {
             score: 0,
             assets,
             screen,
+            timing: Duration::default(),
         }
     }
 
     /// Update the internal state.
-    pub fn update(&mut self) {
+    ///
+    /// # Arguments
+    ///
+    /// * `dt`: The time delta since last update.
+    /// * `controls`: The player inputs.
+    pub fn update(&mut self, dt: Duration, controls: Controls) {
+        let one_frame = Duration::from_secs_f64(1.0 / 60.0);
+
+        // Advance the timer by the delta time
+        self.timing += dt;
+
+        // Step the game logic by one frame at a time
+        while self.timing >= one_frame {
+            self.timing -= one_frame;
+            self.step(&controls);
+        }
+    }
+
+    /// Draw the internal state to the screen.
+    pub fn draw(&mut self) -> &[u8] {
+        // Clear the screen
+        self.clear();
+
+        // Draw the invaders
+        for row in &self.invaders.grid {
+            for col in row {
+                if let Some(invader) = col {
+                    blit(&mut self.screen, &invader.pos, &invader.sprite);
+                }
+            }
+        }
+
+        &self.screen
+    }
+
+    fn step(&mut self, _controls: &Controls) {
         // Find the next invader
         let mut invader = None;
         while let None = invader {
@@ -159,23 +206,6 @@ impl World {
         invader.sprite.update_frame(frame);
     }
 
-    /// Draw the internal state to the screen.
-    pub fn draw(&mut self) -> &[u8] {
-        // Clear the screen
-        self.clear();
-
-        // Draw the invaders
-        for row in &self.invaders.grid {
-            for col in row {
-                if let Some(invader) = col {
-                    blit(&mut self.screen, &invader.pos, &invader.sprite);
-                }
-            }
-        }
-
-        &self.screen
-    }
-
     /// Clear the screen
     fn clear(&mut self) {
         for (i, byte) in self.screen.iter_mut().enumerate() {