1. Because the end faces of matches are square (or nearly square), put five matches together so that the two diagonal points of the end faces of matches face the two diagonal points of other matches respectively, and the gap in the middle is an approximately pentagram.

You can do it directly by hand, but it is easier to use the buoyancy of water in water.

Supplement: If the hollow in the middle is pentagonal instead of pentagonal, that is, five matches are placed directly at a distance of 72 degrees (about 72 degrees is ok), and the edges of the bottom surfaces of the five matches touch each other, the gap in the middle is a regular pentagon. This is the most convenient on the desktop, and it doesn't need to be in the water.

2. Immerse the knotted hair in water, then gently pull the hair at both ends with a little force and let it go immediately. Repeatedly, once tight and loose, the knotted hair will be slowly opened by the tension of water, and the knot will be untied.