By Chris Hogg
BBC News, Tokyo
If someone left an envelope with a crisp 10,000 yen ($80, £41) note in it, would you:
What would you do if you found a wad of money?
A - get on the phone to your favourite restaurant and find out if they have a spare table
B - get down to the bank and deposit it
C - take it down to the nearest policeman and hand it over intact?
If you live in Japan the answer, it would seem, is C.
In recent weeks more than 400 blank envelopes containing 10,000 yen bills have been left in the gents toilets of local council buildings all across Japan.
Also in the envelope were notes asking that the cash be used for "ascetic training".
Then, over the last few days, 18 residents of a Tokyo building found a total of 1.81 million yen ($15,210) stuffed into envelopes in their mailboxes.
This time there was no note telling them what to do with the money.
Half an hour's taxi ride away from the apartments, money was falling out of the sky last week - a million yen floating down from another apartment block above a convenience store.
What is notable about all these events, whether they are connected or not, is the unease they have caused among those lucky enough to find the cash.
And apparently the money has all been handed over to the police.
When we visited the Glanz Ober apartment block, where 18 lucky householders found envelopes stuffed with cash, it appeared deserted.
It would have been easy for whoever was dropping off the money to slip in and out undetected.
We tried to speak to someone who had received the cash, but there was no answer from most of the flats.
The woman in apartment 101 would not come to the door, but through the intercom she told us she had not received the mysterious windfall.
Money came "falling from the sky" outside a convenience store
"I have no idea who did it," she said.
Then we were thrown off the premises. One of the residents, an angry man in his sixties, told us they were fed up with all the attention they had received in recent days.
"People are very, very worried," he told us. "Put yourself in our shoes. We are very anxious."
The Japanese are very private people, and this money seems to be making them nervous.
Some fear it is tainted in some way, perhaps "hot" money from a robbery or some other criminal endeavour.
As we made our way to the location of the second "public donation", our taxi driver rubbished that idea.
"I don't think it's a bad person," he told us. "A bad person would keep it to himself. If you have extra money you might as well donate it, put it to good use."
At the Popura convenience store, the manager Mr Horiuchi was again nervous, and unwilling to give us his first name.
He told us what happened when the bills started falling out of the sky last week. "People all around started collecting it. We collected 96 10,000 yen bills. Then we found four more on nearby rooftops."
He said the honesty of the passers-by was "remarkable".
There are a number of different theories about who might be behind the mysterious donations.
People have speculated that there might be a religious motive.
One of the biggest temples in Tokyo is just a few streets away from the convenience store where the money fell "out of the sky".
But worshippers and visitors there did not think much of the idea that these "good works" were motivated by a desire for greater reward.
"We use money as offerings at temples, and for presents at special events like weddings," one man told us.
But I can't quite see what the significance of leaving it in a toilet would be," he added.
"It tells you that it is a man who is leaving those envelopes," said another visitor to the temple, a woman. "I think it is a rich businessman who doesn't know what to do with his money."
In most cases, the police are looking after the money until its owner, or owners, come forward.
Reports say they have already discounted one claim from a man who told them it belonged to his relative - clearly not everyone in Japan is scrupulously honest.
If it is not claimed within six months, the money left in municipal toilets will become the property of the local council in the town or city where it was found.
That led one visitor at the temple, who works for a municipality, to one bizarre conclusion.
"It must be a foreigner," he told us.
"All Japanese would know that the money if unclaimed would be given to the local council, they would know what their countrymen are like too, so nervous about something like this, they would know that trying to give away money like this is a waste of time."