In a coversation with my five year old son, I made the mistake of
saying 'Well you see, mums and dads know everything' in response to
his surprise at me knowing something only he thought he knew.  His
immediate response was 'Well in that case how many sofas are there in
the world?'  Can anyone offer an informed / educated guess?

Maybe your son (at age 5) has already heard of "The Deeper Meaning of
Liff" by Douglas Adams (celebrated author of the Hitchhiker's Guide To
The Galaxy) and read this snippet on the internet:

Q: I just read The Deeper Meaning Of Liff, and found it to be
uncompleted. What is the word for the feeling you get if you break an
arm during abwong? Is there a definition for Tab? What is the correct
number of sofa's? Really, the book seemed incomplete without this
information.

DNA: I don't know when reading this forum last made me laugh out loud.
I love the idea of the 'correct number of sofas', and that clarity is
the nephew of talent.
http://www.douglasadams.se/stuff/qanda.html

If Douglas Adams has something to do with this question, then I hazard
the answer will be 42, since that is after all the answer to life, the
universe and everything.  Tell your son there are 42 sofas in the
world.  42 is after all quite a large number to a 5-year-old :-)

Well, if there are only 42, I have almost 10% of them.
This may be a good time to revise your claim by adapting it mean that
M&Ds know  (or can ask G-A  ;) anything that can be known, but that
there are questions for which no one knows the answer.  Then you can
talk about putting his question in practical terms that he can
understand:  how many sofas in each dwelling (his answer), how many
dwellings in your town/country, in what other countries people use
sofas, the fact that in many countries people don't use upholstered
furniture, and so on.

This Question defies an exact answer because new sofas are being built
every minute; and old ones are being scrapped every 2 minutes.

However, as of NOW, the EXACT number is 998,874,341.

Take a note of the time NOW and you do the math for a more precise
figure at the time required.

And, as your bright son will have already perceived, the sofa
population is declining rapidly as more and more people go for
beanbags.

I think there is probably a correlation between sofas and televisions,
although I would guess that there are probably slightly more
televisions than sofas (in bars, clubs, etc)
So I would guess that there are about a billion.

"1.3 billion televisions in the world," 
http://www.charismamag.com/a.php?ArticleID=792

"There are more than 1.3 billion TVs in the world,"
http://www.cs.columbia.edu/~hgs/internet/telecom.html

"If you look at the numbers, there are 1.6 billion TVs in the world"
http://www.theglobeandmail.com/servlet/Page/document/v4/sub/MarketingPage?user_URL=http://www.globeinvestor.com%2Fservlet%2FArticleNews%2Fstory%2FGAM%2F20050808%2FRMICROSOFTONE08&ord=1139498093575&brand=globeinvestor&force_login=true

Although probonopublico might have the more accurate figure :-)

All the best

THX1138

What about TVs in the bedroom?

Sooooo cute!

I think it's time to explain to your boy that sometimes Mom & Dad need
to call "experts" for answers --and that you did call the "Expert
Sofa-Counter" and he said there's soooo many sofas in the world, that
by the time he's done counting that the number has changed again.

You can also just say he was out-to-lunch and you left a message. 

When you're driving with your son through an area that shows a couple
hundred houses, point to them and say ..."Wow son!  That sofa counter
has a LOT of counting to do!..."

temos q encontrar a analogia correspondente. não li o livro, claro,
mas teria de ser enorme. aliás impossívelmente enorme. mas a analogia
provávelmente estará lá. (quantas cadeiras existem no mundo por ex.)
a propósito quantas páginas teria um livro q contivesse a resposta a
todas as questões? vou colocar esta ...

That's asking a lot of the kid, expecting him to understand (?)Portuguese.

Please post in English.  G-A rather insists on it.

Maybe it's Brazilian?

Obrigado!

Translation of rui1-ga:
We have to find a corresponding analogy.  I didn't read the book, of
course, but it must have been huge.  Impossibly enormous, by the way. 
But the analogy is probably there.  (How many chairs exist in the
world for example.)  Similarly, how many pages would a book have that
contained answers to all questions.  I'm going to put this . . .

Oh and only questions and answers need to be English.  Comments can be
any old rubbish and mine usually are.