I deal with someone who's a decent person in general but likes to go on the occasional conspiracy theory spiel. These include that 9/11 was deliberately planned by the US government, ancient aliens stuff, and that mermaids are real.

The other people with us are either open-minded to what she says or nod along vaguely. I either nod along vaguely too or pretend to be concentrating on doing something.

What do you do around people like this?

Alternatively, was 9/11 an inside job? Do people think so where you live? We're not American, btw.