I have been working on css for a while, but I was confused why the parent height is not the same as the inner image height, given the following code.

```
<div style="width:200px">
    <img style="max-width:100%;" src="http://localhost:8888/example/06f1e47fbdb03e48203ebfba/66932afa697bdbc76a1a291e/thumb_d90ab4a531969620e2c2a160.jpg">
</div>

```

I was expect that the parent div would expand correspondingly with different image. The fact is that the inner image is 200\*200, and the outer div is 200\*206, and I have no idea where does the extra 6 pixels comes from. **There is are padding for the outer div or margin for the inner image**. Could someone explain why this happens?