SSL - How do you block porn from appearing on your Mac?

I need to block all porn content from my Mac. My young friend uses the computer and I notice that when she is logging out, she clicks out of porn sites that are opened and tells me she doesn't know how they got there. The question I have is can porn sites pop out of nowhere and show their content out of the blue? Please help and excuse my ignorance in this. Thank you.

I'd suggest something like Net Nanny. I haven't used this before, but it's one of the more famous filters and blockers.

As for your other question, it is very rare that porn sites "just pop up" without the user doing something, even if it's clicking on a link that they thought was safe.

Thank you very much! I searched google and found what you recommended is a nice one. And I also find this internet filter for Mac is very popular. Do you know it? Any suggestion?

Um... I don't know. Which filter are you talking about?