When something makes us feel all warm and fuzzy inside, the corners of our mouth turn up before we've even had a chance to think about it.
To celebrate National Smile Week, HuffPost UK Lifestyle are investigating everything about that curious expression we pull when we're happy.
First things first, why do we smile?
We use facial expressions to convey emotions to others. Reading emotions helps us to gauge situations, so smiling is a way of our body telling others we are happy and want them to be happy too.
When we smile and the muscle stretch reaches as far as our eyes, this is known as a Duchenne smile - according to various science journals, a different area of the brain activates the conscious smile to the Duchenne smile, therefore it is extremely difficult to re-create the latter if we don't feel genuinely happy.
According to Mark Stibich, a consultant at Columbia University and contributor at about.com, smiling not only shows others we are happy but actually increases our own happiness by tricking the body into making us change our mood.
Stibich notes that smiling helps us to relax which can in turn boost the immune system, relieve symptoms of stress, and lower blood pressure.
According to Psychology Today smiling also makes us appear more attractive to others.
What makes us smile?
Different things make different people smile, making this question a little more difficult to answer. We asked our HuffPost UK readers what makes them smile to get some clues.
On Facebook Lindsay MacFarlane said: "Fresh sheets, hugs and waking up to sunshine."
Mah Gol said: "A warm kiss by the one I love or a call from someone dear."
Maria Jose Ribeiro Durao said the happy laughter of her beautiful granddaughters and Robin Evans said 'Life of Brian'.
Other readers sent their suggestions in via Twitter...
What can make us smile more often?
It doesn't take much common sense to figure out that doing a job you enjoy and surrounding yourself with people you love will probably make you smile more often.
But if you haven't got the time or energy for a big life overhaul right now, we'd recommend checking out these Instagram accounts for a quick fix.
Photographer William Kass from São Paulo uses fruit and miniatures to create these amazing images:
Doggy Lord Barney, who loves dancing, bacon, hugs and his little sister Happy (a bunny):
The best bulldog ever:
Twins with the best hair in town:
#FlyingStuff: