Okay....this is a pretty "visual" thing, but let's see if we can do this.
You need a plain old plastic drinking straw, unwrapped.
Pinch the bottom of the straw tightly closed about 3/4 of the way down.
Place the straw flat against your chin with the opening just below your bottom lip.
While still pinching the straw closed and keeping it against your chin, blow into the straw.
Once you get that "whistle" sound take your other fingers and pinch the straw closed above your other two fingers.
While still blowing into the straw and making it whistle, keep your fingers pinched on the straw, but start sliding them up toward your lip.
If you can do this while maintaing the sound, it will get louder and decidedly more shrill.
This should entertain everyone at the table under the age of twelve while mortally embarrassing every adult at the table.
One of the adults will invariable tell you to "grow up" which will entertain the under 12 set even further.
And don't think for a minute that I don't know that you are all going to try this.