Pls I'm trying to create a navigation bar with boo strap3 but its not collapsing with the links on smaller screens

<!doctype html>

<!-- Bootstrap CSS -->
<link rel="stylesheet" href="css/bootstrap.min.css">

<title>Hello, world!</title>
    background: #d3d3d3;
    border: 1px solid gray;

My Website
      <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
        <span class="sr-only">Toggle Navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
    <div class="collapse navbar-collapse">
      <ul class="nav navbar-nav">
        <li><a href="">Google</a></li>
        <li><a href="">Yahoo</a></li>
        <li><a href="">Twitter</a></li>
        <li><a href="">Mammal</a></li>
        <li><a href="">WordPress</a></li>

<h1>Hello, world!</h1>
<div class="container">
<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<script src="bootstrap.min"></script>

Pls your immediate assistance will be appreciated