It tops the list of things you should never say to any woman – let alone the First Lady of America.
So a U.S. congressman has understandably landed himself in trouble after making some rather ungentlemanly remarks about the size of Michelle Obama's bottom.
Jim Sensenbrenner went so far as to suggest President Barack Obama's wife was setting a bad example to children because her rear was too big.
The portly Republican, himself a well-known fan of unhealthy snack foods, suggested Mrs Obama was being hypocritical with her 'Let's move' campaign to tackle childhood obesity.
He was overheard making the comments in a mobile phone conversation while waiting at Washington DC's Reagan National Airport.
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Speaking very loudly, the 68-year-old said he had been attending a church auction three weeks ago when a woman came up to him and praised Mrs Obama.
He apparently went on that after listening to the woman he told her: 'She lectures us on eating right while she has a large posterior herself.'
After his comments were reported, a spokesman for Mr Sensenbrenner said the congressman would be contacting Mrs Obama to apologise.
Mrs Obama has not commented on the remarks.
What is his business? He should learn to mind his business.