This cartoon by Chris Riddell from The Observer relates to UK Prime Minister David Cameron's criticism of comedian Jimmy Carr for participating in an aggressive offshore tax avoidance scheme. Cameron called Carr's tax arrangements "morally wrong". However, Cameron is now being called a hypocrite since wealthy Tory donors are said to be involved in similar tax dodges.
The cartoon is based on the proverb "People who live in glass houses shouldn't throw stones", which means that you should not criticize other people for bad qualities in their character that you have yourself. The saying is so well known that it is sometimes shortened to "People who live in glass houses ... ", as in the title of the cartoon.
Cameron is shown in a glasshouse (not the same as a glass house!) with several buckets full of stones labelled "Tax Avoidance Morality", "Executive Pay Curbs", and "Euro Crisis Solutions" (all areas in which Cameron could be accused of hypocrisy). He has already broken several panes of glass and has just thrown another stone (the symbolism here should be obvious!) A big blue carnivorous plant labelled 'Tory Hypocrisy' is growing in the glasshouse (blue is the official colour of the Tory, or Conservative, Party).
There is more than one possible way to interpret this proverb. It could mean that if you live in a house made of glass, you should not throw stones because you might break your own windows (as Cameron is doing). In this case, the message would simply be something like "be careful how you behave because you might do something you'll live to regret". Alternatively, it could mean that if you live in a house made of glass, you shouldn't throw stones (at other people) because they could easily damage your house in retaliation. According to GoEnglish.com, Cambridge Dictionaries Online, and this video (to name but three), the latter interpretation is the correct one—although the cartoonist seems to be playing with both ideas. For the origin of this expression, see The Phrase Finder.
In the proverb, 'glass houses' has a space, suggesting that it refers to 'houses made of glass'. In the cartoon, however, we see a glasshouse (with no space), which is a structure used for growing fruit, flowers, or vegetables. Another word for glasshouse is greenhouse.
People who live in glass houses should undress in the dark.
• Idioms for Idiots! "People Who Live in Glass Houses..." (YouTube)
• David Cameron accused of hypocrisy for attack on Jimmy Carr.. as wealthy Tory backers use similar tax 'dodges' (Daily Record)
• Top Tory donor linked to Jimmy Carr tax scheme (The Guardian)
• Jimmy Carr and David Cameron feel the sharp end of tax (The Telegraph)
• Jimmy Carr's tax deal becomes David Cameron's headache (The Guardian)