US President Donald Trump was greeted with laughter in September when he told the UN that he had accomplished more than "almost any administration" in his country's history. Mr Trump admitted that he "didn't expect" that reaction - but that it was "just fine". It was one example of the many reasons people laugh - and most of them are not because somebody is being particularly funny. Laughter is primarily a form of bonding; we are 30 times more likely to laugh if we are with others than if we are alone. It is an ancient, universal reaction that is not even limited to humans; it has been documented in many animal species, including apes and even rats.