When we find something funny we usually laugh. It can be bought on by other things such as being tickled, inhaling nitrous oxide (laughing gas) or taking drugs. When we laugh a lot we can experience tears of muscle pain as a 'side effect' of laughing.
Laughter can also affect other parts of our bodies and has been shown to be good for the heart. Scientists are not actually 100% sure why this is but the general consenus seems to be that the happier you are the happier you heart is. People who lead stressful, busy lives, are miserable or suffer from depression are more likely to have a heart attack than those who are not.
Naturally we laugh when we find something funny and there are three main theories about what makes us find something funny. One of these is that something becomes funny when it does not have the outcome that we expect. Apparently if we are told a joke we are already using logical thought to follow through on the outcome that we think is going to happen. If this doesn't happen we need to change this thinking and this is where emotional response comes in.
Another theory is that we find something funny when we feel superior and detached from it. This is a bit of nasty sense of humour in this way as it implies that we are able to laugh about someone who has fallen over, or had misfortune or bad luck in some way.
The other theory is that we laugh as a sense of relief. This is often used in films, as the tension is rising a joke or witty incidence may happen just at the moment when we're expecting something else to happen. This gives us a little break from the stress of the tension build up and we are able to laugh partly out of relief at this point.
Laugher has also been shown to be good for your physical health, giving your heart, diaphragm, abdominals, leg and back muscles a work out to boot!