diff --git a/Shared/ExtensionMethods.cs b/Shared/ExtensionMethods.cs
new file mode 100644
index 0000000..aeecc8e
--- /dev/null
+++ b/Shared/ExtensionMethods.cs
@@ -0,0 +1,20 @@
+using Microsoft.Xna.Framework;
+
+// All extension methods on built-in types (of C# or MonoGame) go in this file.
+
+namespace SemiColinGames {
+  static class ExtensionMethods {
+    // Vector2
+    public static Vector2 Rotate(this Vector2 point, float angle) {
+      float cos = FMath.Cos(angle);
+      float sin = FMath.Sin(angle);
+      return new Vector2(
+        point.X * cos - point.Y * sin,
+        point.Y * cos + point.X * sin);
+    }
+
+    // Point
+    public static void Deconstruct(this Point point, out int x, out int y) =>
+      (x, y) = (point.X, point.Y);
+  }
+}
diff --git a/Shared/Geometry.cs b/Shared/Geometry.cs
index deb144a..22a2cfe 100644
--- a/Shared/Geometry.cs
+++ b/Shared/Geometry.cs
@@ -5,16 +5,6 @@ using System;
 
 namespace SemiColinGames {
 
-  public static class Geometry {
-    public static Vector2 Rotate(this Vector2 point, float angle) {
-      float cos = FMath.Cos(angle);
-      float sin = FMath.Sin(angle);
-      return new Vector2(
-        point.X * cos - point.Y * sin,
-        point.Y * cos + point.X * sin);
-    }
-  }
-
   // Math functions that return floats rather than doubles, for convenience.
   public static class FMath {
     public const float PI = (float) Math.PI;
diff --git a/Shared/Shared.projitems b/Shared/Shared.projitems
index a028aa5..832e155 100644
--- a/Shared/Shared.projitems
+++ b/Shared/Shared.projitems
@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)Camera.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)ExtensionMethods.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Textures.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Clock.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Geometry.cs" />