While the replicator is a great tool for drawing lines, nothing makes a smoke trail quite like particles. In this tutorial, we'll cover how to create a particle system for a skywriting effect. Please note that the movies in this tutorial require Quicktime 7 to view.

The source for our smoke will be a simple circle shape, so let's get drawing!

Create the source shape

1. Select the circle tool from the toolbar, or press 'C'.
2. Hold down the Shift key to constrain the circle and draw one about this size:

3. Press the Esc key to exit the shape drawing mode.
4. With the shape still selected, go to the Shape tab of the Inspector and set the shape's Feather to -14.

With our source shape ready, we can go ahead and create our particle emitter.

Create the particles

1. With the shape selected, click the 'Make Particles' button in the toolbar, or press 'E'.
2. In the Inspector, go to the Emitter tab and set the Birth Rate to 150.
3. In the Inspector, go to the Emitter tab and set the Speed to 0.

If you go ahead and press Play, you'll see that the particles just sit there, because we set the emitter's speed to 0. Why? We want to move our emitter around and have the particles stay where they are emitted. Think of it as if you were carrying around a bucket of white paint with a hole in the bottom: as you walk around, you leave a trail of paint. If you stand still, the paint builds up where you're standing. So it's time for us to move the bucket, er...emitter around. We'll do this using the trusty Motion Path behavior.

Animate the emitter

1. Move the playhead to frame 1.
2. With the emitter selected, go to the toolbar and select Add Behavior->Basic Motion->Motion Path.
3. Position the start and endpoints of the motion path slightly off either edge of the project area. If you accidentally miss when clicking on a motion path point, just reselect the Motion Path behavior in the Project panel, and the path will reappear.

4. Option-click on the path to add a couple more points, and adjust them like so:

Now that you have some animation set up for your emitter, go ahead and press Play:

You can see that the emitter follows along the motion path, dropping particles as it goes. The particle "chain" only gets so long, though, as the older particles start dying. Since you used the Motion Path behavior to create the animation, it's easy for you to tweak the path, and you can adjust the speed by changing the behavior's length.

It doesn't look too much like a skywriting smoke trail, though, so let's move on to some emitter styling...