It look like a sculpture of something I cannot figure out of which I saw somewhere, any way this is the Number 8 we will make as an effective bookmark.
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We need a tarpaulin size 3.5x1.5 inches.

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Fold it lengthwise the smoothen the edges.

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Half inch from the bottom cut straight 1/3" before the other side then cut straight up half inch before the top.

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Then cut a small triangle from the middle piece above the center as shown. You are now ready to cut the holes at the other side to create number 8.

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Another view of the output.

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When you followed the instructions properly you should have this bookmark. Otherwise, review where you did not follow.

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Number 0 is shown.

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The other side shows number 8.

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When the book is closed, it looks like a regular bookmark unbeknownst is the number 8 and 0 inside.