Index cards, being little more than small piece of paper, are not exactly ideal building blocks. But with the right procedure, you can still make something that will stand up using only four cards. Index cards are generally thicker than regular paper, allowing them to create a structure that will stand up against wind or pressure from any side. The structure will have four walls, each with two additional pieces extending out of the walls for stabilization.
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Place all four cards in a stack and put them on the table in front of you with the long sides running horizontally.
Make a cut on the right side of the cards, about half an inch from the left edge. The cut should go exactly halfway up the cards.
Flip the cards over so the cut you just made is not in the top-left corner of the cards. Make another cut on the new right-hand side, exactly the same as the first.
Take two of the cards and orient them so that they are next to each other, with the slits on the cards facing each other. Slide the two cards together so that the piece below the slit on each card gets inserted into the slit on the other card. Pull the cards apart until they are 90-degree angles to one another.
Insert the slit on a third card into the unused slit at the end of either of the first two index cards.
Take the fourth card and insert one of its slits into one of the two remaining exposed slits on the structure. This will leave just two exposed ends.
Bend down the small piece of card on the outside of one of the slits, then twist the end of the other card so that its slit will fit into the slit on the first card. Once it is in, release the piece and your structure will be standing.