For Beijing's public toilets, three flies are one too many.
The city government is setting new standards in an attempt to clean up its dirty, smelly public toilets.
And in a series of guidelines, it says the number of flies inside should not exceed two.
The curiously precise hygiene ruling set the internet buzzing yesterday, with critics arguing it was too hard to enforce - and just too trivial in the light of all the other problems facing the capital.
But entomologists defended the guidelines, saying that two was a thoughtfully chosen number.
They said it meant eliminating the human waste that attracts flies and that two flies or less would indicate no fly colony was present.
'It is not only feasible, but laudable to reduce the number of flies down to two,' said Dr Qin Qilian , an expert in insect diseases with the Chinese Academy of Sciences' Institute of Zoology.
The guidelines were posted on the website of the Beijing Municipal Commission for City Administration and the Environment on Monday.
They say no toilet can be rated 'clean' unless it has no more than two flies, no more than two pieces of rubbish on the floor and litter left for no more than 30 minutes. They also set limits on the amount of ammonia and hydrogen sulphide -two chemicals that can result in unpleasant smells - that can be used in toilets.
It is not clear if failing washrooms will be punished and if so, how.
The commission admitted yesterday it did not have enough manpower to ensure there were no more than two flies in every public toilet.
'We have not figured out a way to count the flies and I don't think we will actually do this thing,' a staff member at the commission's media office said yesterday.
'Actually, we aren't the one that proposed the figure. We are confused, too.' She said the guideline on flies was proposed by the Beijing Patriotic Health Campaign Committee under the Ministry of Health, which could not be reached for comment. The guidelines drew plenty of reaction on the internet, most of it negative. Some complained they were the work of irresponsible or incompetent bureaucrats.
'Our officials are gifted to produce regulations that look serious, but are impossible to enforce,' one blogger said on Sina Weibo.
Another wrote: 'One fly would be lonely and three flies could have an affair, so two is the best.
'Our government is very considerate to maintain a harmonious fly society in the public toilets.'
And another said: 'If our good government really gets serious and begins counting the flies, the unemployment rate in Beijing will be reduced to zero.'