Ok, open up your version of flash (I've only used Versions 5 and MX, but I think this tutorial will work for most versions)
Right, now that's over, insert a new layer by clicking the button shown below.
Name one of the layers 'red' and the other 'blue'. To re-name a layer, double click it (you need to double click the text, where it says 'Layer 1' above.
Now draw a blue and red circle in the appropriate layers and spread them to each side of the stage. To switch layers, just left click once on the the layer (where it says 'red' or 'blue'.
Now select ONE of the circles, by clicking it and press F8 and you will get this pop up box:
Name it blue or red, depending on what circle you picked and keep it as a movie clip. Do this with the other circle, changing the name where need be.
Ok, now on the timeline (the bit next to the layers with all the boxes) right click on frame 30 and select insert keyframe. You will need to do this for both layers.
It should look like this:
Now right click again on frame number 29 and 30 of each layer and select Create Motion Tween like so:
The time line layers should now go Purple
Now select the layers at frame 30 and move the circles on the stage to the opposite side of the screen.
for this bit to hide one of the circles while you move the other one
