I saw this greeting card at the souvenir shop at the Science City Museum of Calcutta. I was in standard 3 then. Could not make heads or tails of how it worked. After thinking about it for a considerable long amount of time could come up with a demonstrable idea of the same. It is a simple concept.

There is an envelope that has a sliding image in it. When the card is inside, it is monochrome! And as we slide the image out of the envelope it becomes colored. In the following steps I will demonstrate how to make it.

Step 1: Materials

Few materials will be used to make the magic window.

A colorful image. I took it from my kids coloring book.

OHP slide. Transparency that was used to make slides before the movie projectors were used.

Chart paper. Any card paper that is used to make greeting cards.

Glue + stapler to bind things.

Scissors + cutting knife.

Scale/ruler to measure. Different locales have different names for it.

Cutting board. Not to scar the precious surfaces.

Step 2: Get Started

First cut the image to size. It can be any size. Make sure there are plenty of blank space on top.

Glue it to a card paper to make it stiff. Use ample glue so that to stick it properly.

Now glue the ohp slide to the top. Just a small line of glue. You may also have to staple the slide to the image.

Make sure the ohp can be separated from the image.

Step 3: Marking

Now the secret emerges.

Take a permanent marker and trace the black lines from over the transparency (OHP). Make sure not to mark on any colored part.

Step 4: Make the Envelope

Make an envelope for the image. The envelope will have a secret pocket. The pocket runs through out the envelope.

While inserting the image into the envelope the transparency and the image goes on both side of the secret pocket.

This will make the image invisible and just the transparency visible while inside the envelope.

When the card is pulled out! Both the transparency and the image is visible making them visible as just one image.

Step 5: Finally

The final product will look like this.

Additionally! You can put your own message with the image (not on the transparency). Thus the message will not show when the card is inside the envelope but as you take the card out, the message magically appears. And your viewers will think it as magic.