diff --git a/app/ecs/components/collider.js b/app/ecs/components/collider.js
index de74c66..13f7981 100644
--- a/app/ecs/components/collider.js
+++ b/app/ecs/components/collider.js
@@ -188,8 +188,18 @@ export default class Collider extends Component {
         }
         for (const i in aabbs) {
           const aabb = aabbs[i];
+          const body = this.bodies[i];
           for (const j in otherAabbs) {
             const otherAabb = otherAabbs[j];
+            const otherBody = other.bodies[j];
+            if (body.group === otherBody.group && body.group < 0) {
+              continue;
+            }
+            if (body.group !== otherBody.group || 0 === body.group) {
+              if (0 === (otherBody.mask & body.bits) || 0 === (body.mask & otherBody.bits)) {
+                continue;
+              }
+            }
             // todo accuracy
             if (intersects(aabb, otherAabb)) {
               intersections.push({
@@ -244,6 +254,14 @@ export default class Collider extends Component {
     }
   }
   async load(instance) {
+    for (const i in instance.bodies) {
+      instance.bodies[i] = {
+        ...this.constructor.schema.constructor.defaultValue(
+          this.constructor.schema.specification.concrete.properties.bodies.concrete.subtype,
+        ),
+        ...instance.bodies[i],
+      };
+    }
     instance.updateAabbs();
     // heavy handed...
     if ('undefined' !== typeof window) {
@@ -270,15 +288,14 @@ export default class Collider extends Component {
       subtype: {
         type: 'object',
         properties: {
+          bits: {defaultValue: 0x00000001, type: 'uint32'},
           impassable: {type: 'uint8'},
+          group: {type: 'int32'},
+          mask: {defaultValue: 0xFFFFFFFF, type: 'uint32'},
           points: {
             type: 'array',
             subtype: vector2d('int16'),
           },
-          tags: {
-            type: 'array',
-            subtype: {type: 'string'},
-          },
           unstoppable: {type: 'uint8'},
         },
       },
diff --git a/app/ecs/schema.test.js b/app/ecs/schema.test.js
index 366028e..98f38b4 100644
--- a/app/ecs/schema.test.js
+++ b/app/ecs/schema.test.js
@@ -2,11 +2,12 @@ import {expect, test} from 'vitest';
 
 import Schema from './schema.js';
 
+const compare = (specification, value) => {
+  expect(new Schema(specification).defaultValue())
+    .to.deep.equal(value);
+};
+
 test('defaults values', () => {
-  const compare = (specification, value) => {
-    expect(new Schema(specification).defaultValue())
-      .to.deep.equal(value);
-  };
   [
     'uint8',
     'int8',
@@ -54,6 +55,22 @@ test('defaults values', () => {
   );
 });
 
+test.todo('defaults nested values', () => {
+  compare(
+    {
+      type: 'array',
+      subtype: {
+        type: 'object',
+        properties: {
+          foo: {defaultValue: 'bar', type: 'string'},
+        },
+      },
+    },
+    [{}],
+    [{foo: 'bar'}],
+  );
+});
+
 test('validates a schema', () => {
   [
     'uint8',
diff --git a/public/assets/magic-swords/start.js b/public/assets/magic-swords/start.js
index 7e068ca..1267826 100644
--- a/public/assets/magic-swords/start.js
+++ b/public/assets/magic-swords/start.js
@@ -14,6 +14,7 @@ for (let i = 0; i < N; ++i) {
     Collider: {
       bodies: [
         {
+          group: -1,
           points: [
             {x: -2.5, y: -2.5},
             {x: 14, y: -2.5},