"Oh look, a dead bird!" is the sentence that has made many a person look stupid. The basic idea is to point up into the sky while pronouncing the sentence. Despite the fact that dead birds are incapable of flight (so far as I know), people invariably look skywards expectantly. They may sometimes even ask "Where?" before realising their error!
Why we should be worried
We can therefore say that most of the time we don't give much thought to our actions, we just blindly obey the person telling us to look. This practical joke should be taken as a warning: Always think about what somebody asks you to do! If you think just one second about looking for a dead bird in the sky, you will create a mental image of the dead bird, and realise that it is impossible for the dead bird to stay immobile in the sky (because it is of course possible for a bird to die or be shot down, but by the time you look into the sky the bird is on the ground). This can be applied to numerous situations where people try to trick you.
Actually there is a 1 in 46263467 chance that a dead bird could stay in the sky long enough to look up and see it.
I was once told that if a bird died while its wings were in a certain position, it is technically possible for it to stay stuck in a gliding position, and therefore stay airborne for a considerable amount of time, however I have yet to find scientific proof of this!
And now for something completely different
On a more serious note, if ever you find a dead bird then it may have Avian Flu.
On finding a dead bird you should on no account touch it! According to Defra (Department for Environment, Food, and Rural Affairs):
If you find one or more dead wild swans, ducks or geese (wild fowl); 3 or more dead wild birds of the same species; 5 or more of different species in the same place you should contact the Defra Helpline (08459 33 55 77). Currently the Helpline is available between the following hours Monday - Friday 6.00am to 10.00pm and Saturday - Sunday 6.00am to 10.00pm. You will be asked for details of your finding and its location. See http://www.defra.gov.uk/animalh/diseases/notifiable/disease/ai/ for further information. This is only applicable in the UK of course.