1. Why scissors cut at the crossing point of the two blades, not along the whole blade
Look closely at your hands while cutting paper. At the very tip of the cut, there is a tiny point where the two blades cross. Although the blades are long, the force is applied only around that specific spot. Scissors are tools that cut using "shearing force," which means pushing objects from opposite sides in different directions as the blades slide past each other [1].
In Western-style scissors (the Roman type), the material like paper or cloth is cut as the crossed part of the two blades closes [1]. As you close the scissors, the crossing point moves along the blades. The cut follows the path of this moving point, so the cut line advances little by little. This is somewhat similar to how the interlocking teeth of a zipper move one by one, though a zipper connects things while scissors separate them.
So, why does paper cut when force is concentrated at just one point?
2. Why "sliding" and "tearing" describe the same spot on the blades
The NTT East Japan column explains that scissors concentrate force at the single point where the two blades clamp, effectively "tearing" the object apart [2]. The term "shearing force" used in encyclopedias describes the same event: the action happening at the single point where the blades cross [1]. Although the words differ, both descriptions refer to what occurs at that intersection.
Paper is made of many thin fibers gathered together. It is easier to understand if you think of the fibers breaking one after another due to the force concentrated at that single point, and the cut continuing as the point moves. According to the same column, the sharpness of scissors depends on the grip, the precision of the pivot (the fulcrum) connecting the two blades, and the shape of the blades that transmit force like a lever [2]. Let us look at the blade shape and the lever that sends grip force to the edge.
3. How the slight twist in the blades focuses force on the contact point
According to the Japanese encyclopedia, the blades of Western-style scissors are slightly twisted so that shearing force concentrates at the contact point between the blades [1]. This twist is described as a shape designed to focus force at the point where the two blades touch.
On the back side of the blades, there is also a gap (called "suki") that changes how the blades meet depending on the material being cut [1]. Even though force is concentrated at one point, the required contact style differs slightly for paper versus cloth, or thick paper versus thin paper.
However, the explanation about the twist is found in the encyclopedia and its derivative sites. No manufacturer design documents are cited to support this explanation, so it is best to accept this with the strength of "this is how it is explained."
4. How the pivot works as a lever to send your grip force to the cutting edge
In Western-style scissors (the Roman type), the pivot is located between the blades and the handles. When you squeeze the handles, that force is transmitted to the blade tips on the opposite side of the pivot, acting as a lever [2].
In a lever, force is transmitted more easily when the point where force acts (the point of application) is closer to the pivot. This means cutting closer to the pivot makes it easier with the same hand force. Applied to scissors, cutting thick paper or hard objects near the base of the blades (closer to the pivot) should be easier than using the tips. Here is the flow: the blade shape (twist) creates a single contact point; the grip force is sent to the edge via the pivot lever; and as the blades close, the point moves, advancing the cut. But when did this shape first appear?
5. How the X-shaped Roman scissors with a middle pivot date back to 27 BC
Iron relics of scissors with an X-shape and a pivot in the middle (the Roman type) date back to 27 BC in the Roman era [3]. In China, they appeared after the Tang Dynasty, and the gold-copper scissors preserved in the Shosoin in Japan are also said to be of this type [3].
The oldest relics of scissors are iron ones made in Greece around 1000 BC. These were of a type where the U-shaped part acts as the pivot, and they are said to have been used for shearing wool [3]. They are said to have existed in the Bronze Age, but the inventor and exact date cannot be stated definitively.
In Japan, they are said to have been introduced via China around the 6th century, and the oldest scissors in Japan were excavated from a 7th-century burial mound [3]. Until the Edo period, the main type was the Japanese-style scissors (wasami) with a U-shaped spring acting as the pivot [3]. Western-style scissors began to be imported and produced in large quantities from the Meiji period [1]. The modern form of scissors, said to have been created by Fiskars around 1880, has a basic structure that reports state has not changed in over 140 years [4]. Note that this is a separate date from the Roman relics.
6. How scissors are used to measure the toughness of materials, and what 2005 has to do with it
Scissors are used in a method to measure load and displacement to study the "toughness" (fracture toughness) of materials [5].
However, a 2005 paper by Atkins and Xu states that an analysis explaining how force changes according to the opening angle of the blades had not existed before, and this paper presents that theory [5]. Although the basic structure has remained similar for about 2,000 years, a theory explaining force changes associated with blade opening was presented in a 2005 paper. This article does not include the equations or numerical values. However, the fact that "force changes with the opening angle" itself is the starting point of the research. Everyday tools seem obvious, but when measured precisely, questions remain. Scissors are a good example of this.
7. How to test the crossing point and the base of your scissors with scrap paper
Prepare a pair of scissors at hand and some scrap paper. Sit down and ensure there are no people around you. Do not point the blade tips at anyone. First, cut slowly and watch the tip of the cut. Can you see the point where the two blades cross moving as the cut progresses? Next, compare cutting thick paper near the tips of the blades versus near the base (close to the pivot). If the feel is different, it matches the lever explanation [2]. If it feels the same, factors like paper type or blade condition are also possible.
Regarding cutting technique, the Ministry of Education, Culture, Sports, Science and Technology's art instruction materials state that you should hold the scissors perpendicular to the paper and not tilt the blades. It also notes that when cutting curves, you should turn the paper rather than turning the scissors [7].
For safety, always sit while cutting and never point the blade tips at people.