If it is light hearted stuff then I join in...... and expect some banter back. This is a good way to become 'accepted' by my son's friends and let them know that they can have a laugh even with an 'oldie' in the car.
I do tend to 'cough' if a swear word is uttered (followed by a steely glance in the mirror)... just to let them know it had been picked up and wasn't allowed in the car.
If the conversation seems more serious ... then I tend to be mute unless asked for my opinion.
Most unexpected question from an 11 yr old....
If 'A' is going out with 'B' but they haven't kissed can I ask her out?
Best discussion so far was..... how many countries/ cultures would have to change their diet if we didn't have bread/ flour..... and what would they eat instead.