diff --git a/js/behaviours/main.mjs b/js/behaviours/main.mjs
new file mode 100644
index 0000000..2768637
--- /dev/null
+++ b/js/behaviours/main.mjs
@@ -0,0 +1,25 @@
+import * as entity from "../entity.mjs";
+
+
+
+export const behaviours = new Map();
+
+export function exists (name) {
+	return behaviours.has(name);
+}
+export function get_tick (name) {
+	return behaviours.get(name).tick;
+}
+export function init (enemy, name, params) {
+	entity.get_meta(enemy).set("behaviour", name);
+
+	const behaviour = behaviours.get(name);
+	if (behaviour.hasOwnProperty("init")) {
+		behaviour.init(enemy, params);
+	}
+}
+
+
+
+import sine_x from "./sine_x.mjs";
+behaviours.set("sine_x", sine_x);
diff --git a/js/behaviours/sine_x.mjs b/js/behaviours/sine_x.mjs
new file mode 100644
index 0000000..4aac719
--- /dev/null
+++ b/js/behaviours/sine_x.mjs
@@ -0,0 +1,34 @@
+import * as entity from "../entity.mjs";
+import { underride } from "../underride.mjs";
+
+
+
+export default {
+	tick: (enemy, deltatime) => {
+		const {y} = entity.get_pos(enemy);
+
+		const meta = entity.get_meta(enemy);
+		const age = meta.get("age");
+
+		// `movement_speed` is in hertz and `Math.sin` input is in radians.
+		const age_factor = meta.get("movement_speed") * Math.PI * 2;
+		const x = meta.get("center_x") + Math.sin(age * age_factor) * meta.get("movement_scale");
+
+		entity.set_pos(enemy, x, y);
+	},
+	init: (enemy, params = {}) => {
+		params = underride(params, {
+			movement_scale: 100,
+			movement_speed: 1/4,
+		});
+
+		const {x} = entity.get_pos(enemy);
+
+		const meta = entity.get_meta(enemy);
+
+		meta.set("center_x", x);
+
+		meta.set("movement_scale", params.movement_scale);
+		meta.set("movement_speed", params.movement_speed);
+	},
+};
diff --git a/js/enemy.mjs b/js/enemy.mjs
index d9122e6..7b46b0a 100644
--- a/js/enemy.mjs
+++ b/js/enemy.mjs
@@ -1,14 +1,11 @@
-import { assert } from "./test.mjs";
-
 import * as entity from "./entity.mjs";
 import { register_tick } from "./tick.mjs";
+import * as behaviours from "./behaviours/main.mjs";
 
 
 
 const enemies = [];
 
-const behaviours = new Map();
-
 function tick_enemy (enemy, deltatime) {
 	const meta = entity.get_meta(enemy);
 	const age = meta.get("age") + deltatime;
@@ -16,8 +13,9 @@ function tick_enemy (enemy, deltatime) {
 
 	if (meta.has("behaviour")) {
 		const b = meta.get("behaviour");
-		if (behaviours.has(b)) {
-			behaviours.get(b)(enemy, deltatime)
+		if (behaviours.exists(b)) {
+			const callback = behaviours.get_tick(b);
+			callback(enemy, deltatime);
 		}
 	}
 }
@@ -32,8 +30,8 @@ export function get_all () {
 	return enemies;
 }
 
-function create () {
-	const enemy = entity.create(100, 100, 40, 40);
+function create (...args) {
+	const enemy = entity.create(...args);
 
 	const meta = entity.get_meta(enemy);
 	meta.set("age", 0);
@@ -41,15 +39,10 @@ function create () {
 	return enemy;
 }
 
-const e = create();
+function set_behaviour (enemy, behaviour, params) {
+	behaviours.init(enemy, behaviour, params);
+}
+
+const e = create(300, 100, 40, 40);
+set_behaviour(e, "sine_x");
 enemies.push(e);
-entity.get_meta(e).set("behaviour", "sine_x");
-
-behaviours.set("sine_x", (enemy, deltatime) => {
-	const {x, y} = entity.get_pos(enemy);
-
-	const meta = entity.get_meta(enemy);
-	const age = meta.get("age");
-
-	entity.set_pos(enemy, 100 + Math.sin(age) * 80, y);
-});
diff --git a/js/underride.mjs b/js/underride.mjs
new file mode 100644
index 0000000..8e68028
--- /dev/null
+++ b/js/underride.mjs
@@ -0,0 +1,30 @@
+import { assert } from "./test.mjs";
+
+
+
+function shallow_mutable_copy (source, target) {
+	for (const [key, value] of Object.entries(source)) {
+		target[key] = value;
+	}
+}
+
+export function underride (object, template) {
+	const result = {};
+	shallow_mutable_copy(template, result);
+	shallow_mutable_copy(object, result);
+	return result;
+}
+
+
+
+assert("underriding works.", _ => {
+	const object = underride({
+		gay: true,
+		trans: true,
+	}, {
+		trans: false,
+		invisible: false,
+	});
+
+	return object.gay && object.trans && (object.invisible === false);
+});