I am curious about nth child selector in css. But is hard to understand its logic for me now. Below given example highlight first and the last span.

```html
<div>
<span>This span is limed!</span>
<span>This span is not. :(</span>
<em>This one is odd. </em>
<span>Sadly, this one is not...</span>
<span>But this one is!</span>
</div>

```

```css
span:nth-child(2n+1) {
background-color: green;
}

```