Show tooltip when user moves mouse on column name of bootstrap-vue table

I have a bootstrap-vue table that looks like this;

Here’s the code for the table;

<template>
  <div>
    <b-table striped hover :items="items" :fields="fields"></b-table>
  </div>
</template>

<script>
  export default {
    data() {
      return {
        // Note 'isActive' is left out and will not appear in the rendered table
        fields: [
          {
            key: 'last_name',
            sortable: true
          },
          {
            key: 'first_name',
            sortable: false
          },
          {
            key: 'age',
            label: 'Person age',
            sortable: true,
            // Variant applies to the whole column, including the header and footer
            variant: 'danger'
          }
        ],
        items: [
          { isActive: true, age: 40, first_name: 'Dickerson', last_name: 'Macdonald' },
          { isActive: false, age: 21, first_name: 'Larsen', last_name: 'Shaw' },
          { isActive: false, age: 89, first_name: 'Geneva', last_name: 'Wilson' },
          { isActive: true, age: 38, first_name: 'Jami', last_name: 'Carney' }
        ]
      }
    }
  }
</script>

This is what I want to do. When user moves his mouse to First Name column name cell, I want to show a tooltip that says “Click to sort First Name”.

I am using vue v2.6