Figure A
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Hereโ€™s an update of a Victorian plaything. Cut out the fish on the dotted line and float it on a pan of water. Place a single drop of olive oil in the circle. The oil quickly spreads out the slit and across the water. The fish โ€œswimsโ€ in the opposite direction, like an exhaust-spewing rocket subject to Newtonโ€™s third law of motion. Sadly, the soggy paper fish is only good for just a single use (Figure A).

Now try this new, more durable version:

Figure B

Find a flexible lid from a margarine or yogurt container. Look for the recycling symbol 2 or 4 for low- or high-density polyethylene. (PE is one of the few plastics that floats.) Use a paper punch to make a small circular hole, then cut out the โ€œrocketโ€ shape as shown in Figure B.

Float the rocket in a pan of clean water. Dip the tip of a toothpick in detergent and momentarily touch it inside the rocketโ€™s round hole. As the detergent dissolves, it spreads down the slit and out along the surface of the water โ€” the rocket shoots forward! Touch it again. After a time or two, youโ€™ll have to change the water for the effect to work again.

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Another force is also at work: the Marangoni effect, the difference in surface tensions created by the molecules of detergent as they make the water slipperier and โ€œwetter.โ€ The surface tension is reduced behind the rocket, causing the water in front to contract, pulling the rocket forward (Figure C below).

These tensions, forces, and actions all exist at the single-molecule-thick surface of the water โ€” similar to the two-dimensional world in Edwin Abbottโ€™s Victorian-era book, Flatland: A Romance of Many Dimensions.

Figure C