The figure-eight coil becomes a circle.

1, drop a few drops of soap in the middle of the circular ring and support the circular ring with outward surface tension (provided that the cotton loop must float on the water);

2. Wrap the cotton thread around the rolling pin and put it into the water. Because the cross section of the rolling pin is round, the cotton ring can only be round (provided that the cross section of the rolling pin must be round);

3. Bend a metal wire into a circle, carefully stick the cotton roll on the metal wire, and then put it into the water (provided that the metal wire is hard enough not to deform easily and there is waterproof glue);