A design student has created a hat which is guaranteed to make you smile -- because it will poke a spike into the back of your head if you don't.

Lauren McCarthy's "Happiness Hat" is described as a wearable conditioning device that detects if you're smiling and provides pain feedback if you're not.

While it looks like any other wooly hat, it hides a sinister secret, an arm which reaches down your face detects if you are smiling and if your not it inflicts pain.

If the smile sensor detect your smile is turning into a frown it send a message to a battery-powered servo motor which moves a metal spike into the back of your head.

The less you smile, the more painful the spike is said to get - so this is how Santa keeps his elves looking so happy.