Creating a Mask
A mask will limit the view of the layers associated with it. It allows you to place layers above others which can appear like they are below. It also allows you to cut objects so that only portions of them are visible. You will only see what is masked.

Test it out on the right==>

1. Open a new movie. Make the background black (MODIFY>MOVIE>Then select black from the drop down color box.
2. Rename Layer 1 SKY by double clicking the words Layer1 and typing in the new name. In frame 1 the SKY layer place a large blue square using the rectangle tool. Right Click on Frame 40 and select INSERT FRAME. Lock the SKY layer.
3. Right click on the SKY layer and insert new layer. Rename layer 2 MOTION by double clicking the name layer 2 and typing in MOTION.
4. Draw an odd shape in frame 1 of the MOTION Layer. Select the odd shape you just drew and click the insert menu and select CONVERT TO SYMBOL.
5. RIGHT Click on Frame 15 of the motion Layer. Select INSERT KEY FRAME. RIGHT Click on Frame 25 of the motion Layer. Select INSERT KEY FRAME. RIGHT Click on Frame 40 of the motion Layer. Select INSERT KEY FRAME.
6. Move your Symbol 1 in Frame 1 on the MOTION Layer to the right side of your stage. Move your Symbol 1 in Frame 15 of the MOTION Layer to the left hand side of the stage. Move your Symbol 1 in Frame 25 to the bottom left hand side of the stage. Click in Frame 40 and Move your Symbol 1 to the right bottom side of the stage.
7. Click on Frame 1 of the MOTION Layer. Select the Frame Panel. Go WINDOW=>PANELS=>FRAME.
8. On the Frame panel, select Tweening = MOTION. Do the same for frame 15 and 25.
9. Click on Frame 1 of the MOTION Layer. Select the Frame Panel. Select CW or CCW from the Rotate drop down list. Assign it a number 2, 3, or 4 in the times text box. Do the same frames 15 and 25.
10. Lock The MOTION Frame.
11. Right click on the MOTION Layer and select INSERT LAYER.
12. Rename Layer 3 MASK by double clicking the words layer 3 and typing in MASK.
13. Draw the shape of a window pane and only the glass pane on this layer which extends from frames 1 to 15. You can use the window pane I used, then rotate it three different ways to get the look of a window pane. You would import or copy this into flash. Choose the graphic, copy it three more times, then select MODIFY>TRANSFORM>Then select Flip Vertically or Horizontally.
14. Right click on the word MASK on the MASK layer and select the option MASK. Immediately you will see the MOTION layer associate itself with the mask and both layers will lock.
15. Grab the SKY layer with your LEFT mouse button. Holding the button down drag it till it touches the bottom of the MOTION Layer and associates itself with the MASK. The association is indicated by the indent of the layers .
16. Play your movie.