A spring box is an assembly of a box body and a spring, the box body being formed of a single piece of foldable material such as card stock. The box body bears a message on a message panel. When the box body is closed the message is covered by a cover panel. The box body includes a tab that releasably holds the box body closed with the cover panel overlapping the message panel with the spring biasing the message panel against the cover panel. When the tab is released, the spring suddenly opens the box body, thus to push the message panel upward. The upward movement of the message panel causes the cover panel suddenly to move away from the message panel, thus to expose the message to view. During the sudden opening of the box body, the message panel and the cover panel pivotally move in opposite arcuate directions.
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