Why does a dog wag its tail?
Because a dog is smarter than its tail.
If the tail were smarter, the tail would wag the dog.
The above part looks funny but it’s true and it is very well presented in a classic film Wag the Dog. I usually suggest this film to all my friends, but they rarely find and watch it. But today, I found the movie in youtube and therefore I am uploading all the links here 
more at: IMDB
more at: WikiPedia
