[Vuejs]-Vue.js not work as expected

app = new Vue({ //not vue, it is Vue
    el: "#app",
    data: {
    product: "Boots"
    }
    })

<script src="https://unpkg.com/vue@2.5.16/dist/vue.js"></script>
<div id="app">
    <h2>{{product}}</h2>
</div>