[CARTOON] Tar Balls

Why is it everytime I see a picture of a tar ball, there is a yellow ruler next to it?

<center><br /><a href="http://pastexpiry.blogspot.com/" target="_blank"><br /><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgpkV_8ES6J3Xche-ErWqGhF3Rn_EJTzJxgiq7rfFSZEghnDCy-mFSee-xglHM76c3EDvNt1N2Dv2ywxywNQElOEdqkLRGnNzDRCPBa-Y5ahnk0LMf1RmarZCZbtcrf-Fmk7l1Q-wfUwRog/s320/toon1019+tar+balls.jpg" title="[CARTOON] Tar Balls. images, pictures, cartoon, environment, news, oil, pollution, tragedy, US, " border="0"></a><br><br />Courtesy of <a href="http://pastexpiry.blogspot.com/" target="_blank">Past Expiry Cartoon</a></center><br />
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