Blowing over a blade of grass stretched between your thumbs makes a high-pitched squealing noise that often attracts predators. Done long enough, it can lure animals such as mountain lions, coyotes and lynx close. It can also keep bored kids busy in the back yard for quite a while.
1.)Select a blade of grass. Thin blades of grass work best, but they need to be flat, not round. Very wide blades of grass that have a crease don't work as well either. In other words, bermuda grass works great, but be sure to use the blades, not the stems.
2.)Hold one end of the blade of grass between the tips of your thumbs. With your thumbs slightly bent, press the bases of both thumbs together, catching the other end of the grass between them.
3.)Carefully straighten your thumbs out to pull the blade of grass taut. If you pull it too taut, it will snap and you'll have to start over.
4.)Purse your lips slightly, put them right up against your thumbs and blow so that the air goes between your thumbs and over the blade of grass. It should make a high-pitched whistle sound.
Alternate Method
Get two pieces of grass of the same width and length, or take a thick piece of grass and fold it.
Hold the two pieces of grass on top of each other using the thumb and forefinger of each hand.
Stretch the blades of grass taut.
Blow between the two pieces of grass.
Relax or stretch the blades of grass as needed to make the best sounds.
as I said the tutorial is not by me... so the pictures are obviously not mine either...