Never disabuse friends of their ghost.
I once went to a friend's house which was an old brick house with heating made by black iron pipes hanging in mid-air through which hot air was ducted. The kitchen, they said, had a ghost that would rap on the table when they were cooking.
I looked at the large, heavily varnished, butcher-block table against the wall. I took a gander under it.
"Would I be right in saying the ghost knocks again about an hour or so after you are done with dinner?" I asked. They were stunned and said it did.
So I told them that I thought it was more likely that the pine table, which was varnished only on the top, was expanding as the steam from the cooking made the air moist. The table was bolted by a large steel bracket to the wall which would grip and grip and grip and then let go, rather like a tectonic slip-strike fault, making a sharp noise. When the air dried out again after dinner, the table would shrink with the same effect. I said they could exorcise the ghost with a wrench by loosening the nuts on the securing bolts a turn.
Never saw people I cared about look so sad so quickly before. Lesson learned. Now I reckon I would say "Wow!" and shut the fuck up.