title: What is the purpose of this.props.onChange()?
body: <p>From the react quickstart: <a href="https://facebook.github.io/react/docs/lifting-state-up.html" rel="noreferrer">https://facebook.github.io/react/docs/lifting-state-up.html</a></p>
  
  <p>The purpose of the line:</p>
  
  <pre><code>  handleChange(e) {
      this.props.onChange(e.target.value);
    }
  </code></pre>
  
  <p>was never explained and I can't figure out what it <em>does</em>. Is onChange a built in method of props? I thought props was simply just parameters for components, why can it invoke functions?</p>