test.js

class Parent extends Component {
  state = {
    counter: 0
  }
  handleClick = () => this.setState({ counter: this.state.counter + 1 })
  render() {
    return (
      <div>
        <p onClick={this.handleClick}>{this.state.counter}</p>
        <Child />
      </div>
    )
  }
}

class Child extends Component {
  render() {
    return <div>I will re-render every time the counter changes, even though I do not change myself.</div>
  }
}

class PureChild extends PureComponent {
  render() {
    return <div>I will only ever be rendered once.</div>
  }
}