How come they can give it free?

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How does a site that gives people a free domain name earn money? I got this smartdots site and it's actually free. You don't need to purchase and anything like that to get the free domain name you want. And now I'm curious, that my page is there, they help me out build a webpage like this http://wwww.gotchonline.net.tc without asking anything in return. I don't even click any ads. Please enlighten me about sites that gives complete free stuff like that. Thank you.