What would make you click on an advertisement?

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Is it the persuasive words? The colorful images? Or you just click based on your need? I guess internet users are very smart now that is why they don't just click away in a the sight of an advertisement. The ads on my blog have not been clicked even a single time. That is why I don't earn anything from Adsense. I want to understand what would make my readers click on the ads so I can earn money from it through PPC. What do you think?