Have you ever been in a Paid to Click Site?Do they really pay you?

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I've been lately introduced by a friend to these PTC sites. I'm a student and I badly need some extra income, so I am interested by the idea of earning meaning by just viewing some ads, or reading some e-mails etc. But my question is, are they really legit? Do they really pay you? I mean, i don't want to waste my time on these, expecting I'll get paid only to find out I have been fooled. Thank you!