How do you wolf whistle without using your hands?

Question

I can wolf whistle using my hand/s, but i saw someone on tv do a really good one to round up sheep with a sheep dog and he didn't use his hands he just used his mouth.

How the heck do you do it without using your hand/s?

Answer 1

Here you go.

http://www.wikihow.com/Wolf-Whistle

/rtm