You can use this tutorial to make weeds or grasses swaying in the wind as above. Or colour them orange, yellow and red and you’ve got flames. They are also a useful decorative feature that seems to be quite common these days.
Start with a line…
Go Filter > Distort > Zig Zag, or Effect > Distort & Transform > Zig Zag, and check the preview box so you can see what you are doing. The one above has 10px size, Absolute, 4 Ridges per segment and Smooth Points, but it’s up to you.
So you have your wavey line. Copy it (Cmd/Ctrl -C), Paste in Front (Cmd/Ctrl -V), and select the Rotate Tool (R). Now click on one of the ends of the wavey line and drag the other end up or down a bit (see above).
Now you have to connect the two lines so you can apply a fill. With your Direct Selection Tool (A) select the two end points in the apex by dragging a select over them so they look like the last example above. Then a simple Object > Path > Join (Cmd/Ctrl -J) (select Corner, of course) and you have your shape. Finally, add gradients and transparencies.