How to whistle with 2 fingers?

How do you whistle loudly with two fingers in your mouth?

• Some people can really whistle loud enough in this manner to be heard from quite a distance. How can I do it? I know how to whistle without the fingers. Thanks.

• Answer:

  I know how to do it in principle, but I've never successfully done it myself. The idea is to make a very small space for the air to pass through. If the space is small enough, it will whistle. So I think you just somehow create a very small passage between your fingers and your tongue. I suspect this is one of the kind of things that you can just experiment with and then when you get it once, you know how.