c++ - Why is std:: used by experienced coders rather than using namespace std;?

Possible Duplicate:
Why is 'using namespace std;' considered a bad practice in C++?

The other day when I asked a question someone replied saying if someone asks a question, show them the right way to do it instead of using namespace std; which I thought was a bit weird, as using namespace std; is way easier, But I guess I'm failing right now as I am a 'beginner' coder and you guys know better.

So I guess my question is:
Why std:: instead of using namespace std;?

Thanks.