How do you block porn from appearing on your Mac?

I need to block all porn contents from my Mac. My young friend uses the computer and I notice that when she is logging out, she clicks out of porn sites those 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 contents out of the blue? Can I get the internet filter for Mac to block unwanted sites? Please help and excuse my ignorance in this. Thank you.

Porn sites can popup due to an adware on your computer. You can get any filter to block the traffic.