We don't do it when we think we do. It's been found that if you ask people what makes them laugh they'll talk about jokes and humour, but we laugh most frequently when we are with other people - and hardly ever at jokes.
It's a social emotion and we use it to make and maintain social bonds.
We also make very strange noises when we laugh - from wheezes and squeaks to gasps and snorts - and each sound simply reflects the muscles in the chest squeezing out air from our ribcages under very high pressures.
