How does AdSense works?

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I signed up at Adsense and my blog already has some ads posted on it. It has been almost three months now but I haven't earned a centavo. Why is that so? Do you have AdSense on your blog too? Did it work for you? Do you have there any advice on how to make money using this? I have considered plugging my blog to every networks that I have joined, too. Doesn't work.