One of the basic steps in form perception is the grouping of features that belong to a single object. Gestalt psychology provides a set of principles (sometime called "laws" but that is probably too strong a term) for which items are grouped together. For example, the principle of proximity states that, if all else is equal, items that are close together will be perceptually grouped together.
One of the gestalt principles of grouping is called common fate. Common fate states that, if all else is equal, items that undergo the same change will be perceptually grouped together. The change can be any perceptually apparent attribute -- a change in color, a change in size, a change in location will do.
Look at the jumbled lines in the figure below. Can you see the hidden message in the lines? We can use common fate to make the message appear. If we move all the lines in the message in the same way, the perceptual system will group them together and the message will be apparent. Click on the "Move It" button to see common fate in action. Click the button again and the message will disappear when it stops moving.
You can also simultaneously change the color of all of the elements that make up the message. Doing so should allow common fate to group the changing elements together to reveal the message. Finally, you can make all the elements that make up the message grow and shrink in size. Again, common fate should group the elements that are changing in the same way and the message should appear.