NASA's Earth Observatory frequently posts a single image file that shows both a before-and-after image of the same place. Follow this technique to create two separate images from the single file so you can put them into a stack. Go to Earth Observatory's before-and-after images of springtime flooding in Argentina

1. Read through the story to learn about the events responsible for the differences in the two images.
2. Download the single image file that shows both images. Save it in an obvious place such as your Desktop or Downloads folder.
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   Right-click on the image (Mac users press ctrl-click) and choose the option your browser offers to save the image to your own computer.
   
3. Launch ImageJ and open the file from within it.
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   From the ImageJ menu, choose File then Open... then navigate (browse) to where you saved the image.
   
   
4. From ImageJ's toolbar, choose the rectangular selection tool. Place your cursor on the image and drag a selection box over the top picture. Use the arrow keys to move the selection into the upper left corner of the image. Use the handles to adjust the selection size to include the date below the image.
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5. From ImageJ's menu bar, choose Image --> Duplicate... and accept the default name.
6. Click the large image again to activate it. Click inside the yellow selection rectangle and drag it down over the bottom image. It's essential that the selection rectangle remains the same size.
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7. Choose Image --> Duplicate... and accept the default name again.
8. Once you have the two images in separate windows that are the exact same size, activate the large image again and close it.
9. From ImageJ's menu bar, choose Image --> Stacks--> Convert Images to Stack
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   Download the stacked images ( 1.1MB Apr10 07). Open the file from within ImageJ.