AdSense -- Am I missing something here?

The fact that you're here, reading this blog tells me that you're a blogger

yourself, so you must've heard of this. Blogger has this nifty new feature called AdSense. Here's the deal; you add a bit of HTML to your template, and these ads, relevant to the current topic, show up in the sidebar of your blog. Now, if a person were to browse through your blog and to click on one of these ads, it would make you money. Yes, real money. At least, so they claim, as you can see on the picture on the right. And if we are to believe

them, the amounts of money involved are considerable. The example given by Google itself, as can be seen on the image on the left, is

over $900,-. So, here's the part that I just do not get. Imagine, if you will, a blogger with not much to do all day. So, to kill some time, this hypothetical blogger adds these hypothetical ads to his hypothetical blog.

What is to stop him from incessantly clicking these hypothetical ads all hypothetical friggin' day, thereby earning himself some hypothetical cash?

I must be missing something mindnumbingly obvious here (some shit about unique visitors, probably), but I can't stop thinking about it.

But then, I'm not too bright a feller.

Cheers