Your son bought the piano.
Good one. You really are a sharp tack aren’t you?
You might try asking the piano why it’s there. Unless you’re scared to…
Have there been any mysterious fishy smells in the house?
You might have a piano tuna…
As far as how the piano got there–if it’s a Clavinova then it could be one of a few different sizes/weights. Some of these would not be that difficult to transport.
Okay, that’s it. I will pay you guys not to post in a thread that I start. How much do you want? 
At least a grand.
Beating? Quite the temperment.
Someone just isn’t being upright.
I was going to say that it’s not so amazing for a piano to slip into “A flat,” but Cairo Carol said the piano appearance happened in a house.
I wonder if we’re going to have another thread about somebody coming home from a vacation, and finding the piano is gone, perhaps replaced by a sofa-sized heap of car keys?
G-d, I’ve been good…
Okay, that tops the time I came home from vacation to find a strange pair of filthy white althetic socks and a leather whip in my otherwise undisturbed apartment.
We’ll be with you in a minuet…
You can tune a piano, but you can’t tuna fish, unless it’s in the right scale.
Of its own achord?
Ah-hah. We have an answer, which while disappointingly consistent with the known laws of the universe, is not what I expected.
It turns out that our former tenant, who moved out permanently about a month before my husband and son arrived, recently bought the piano. I’m quite surprised. She’s rented from us during the school year for 3-4 years - I know her slightly and she’s never mentioned any interest in playing. I’m also a bit amazed that she’d make that kind of investment, as those Clavinovas are hugely expensive (at least by the time they get shipped to the Big Island, where our house is), and she’s been having trouble paying her rent.
Wait, maybe those two facts are connected …
Very strange also that she’d spend all that money and then not take it with her when she moved out for good. I guess she’s going to make arrangements to pick it up, just hasn’t done so yet.
Now that I’ve satisfied everyone’s curiosity I’m going to find out (I hope) what was in the vase Autolycus sniffed.
You can’t make up for quality with volume.
Just not your forte…
Did ya hear about the piano study piece for students that was themed on the assassination of Julius Caesar?
It’s the Etude Brute…
Maybe it stopped in to tinkle.
Are there any circumstances in which we are allowed to do bodily harm to other-I will not say “fellow”-Dopers?