Trigger animations work with PowerPoint 2002 and above. In this exercise, we'll look at a basic trigger animation. You can download the sample presentation here... (around 75 kb)

You'll need two images to illustrate the trigger animations - I've used images downloaded from Microsoft Office Online's Clipart section.

1. Create a new presentation (Ctrl + N) and insert a blank slide.
   
   
2. Insert two images of the same size within this slide - choose Insert | Picture | From File. Keep both of them in different areas of the slide.
   
   
   
   
3. Right-click one of them and choose Custom Animation from the resultant menu. This will activate the Custom Animation task pane.
   
   
   
   
4. Click the 'Add Effect' button and then choose an Entrance animation. I've chosen the Appear animation since that provides the most optimum clickover effect.
   
   
   
   
5. By default, PowerPoint chooses "On Click' for the Start options. That's what we want so don't alter that.
   
   
   
   
6. Below that option, you'll find the name of your image in a listing of animations. Besides the name, you'll find an arrow that reveals a drop-down menu on being clicked. Choose the Timing option in that menu.
   
   
   
   
7. This will open the Timing tab of the animation dialog box. Click the 'Triggers' button so that more options are visible. Choose the second option that reads 'Start effect on click of:' and choose the name of the second image we inserted within the slide. Click OK.
   
   
   
   
8. Place both the images one on top of the other (that's why we chose images of the same size) and play the presentation. Clicking on the visible image should show us the other image. If that doesn't work, right click the image that's visible and choose Order | Send To Back (or Send Backward multiple times depending on how many objects you have onscreen).
   
   
   
   
9. Save your presentation.