Why is a div with “display: table-cell;” not affected by margin?

I have div elements next to each other with display: table-cell;.

I want to set margin between them, but margin: 5px has no effect. Why?

My code:

<div style="display: table-cell; margin: 5px; background-color: red;">1</div>
<div style="display: table-cell; margin: 5px; background-color: green;">1</div>