Magical contrast: Why can some people not open bottle caps but split watermelons with their bare hands?

In the eyes of many people, the strength of girls is a mystery. I still remember that my high school classmate always asked me for help when she couldn't unscrew the bottle cap. Every time I helped her, I would laugh at her. But one day, we bought a few watermelons for the class activity. When everyone was worried about not buying a fruit knife, she split the watermelon with one palm, leaving us shocked on the spot and the reputation of a powerful Vajra Palm. Have you ever encountered similar situations in your life? Is it true that girls can't unscrew the bottle cap because they are weak? Then why can she split the watermelon with her bare hands? Could it be that she is interested in me and deliberately can't open the bottle cap to get close to me? Wake up, don't dream! Today, let's talk about the reasons behind this magical contrast.

Have you ever experienced the pain of not being able to open the bottle cap? Image source: WeChat public account @Global Physics

01Sealing principle of bottle cap

First, let's disassemble the bottle cap. You can see that the bottle cap has the top, sealing gasket, thread, connecting bridge and anti-theft ring. When the beverage leaves the factory, the bottle cap and the anti-theft ring are connected by a row of structures called connecting bridges . Once the bottle cap is unscrewed, the connecting bridge will break. Through it, we can determine whether the beverage has been opened.

Image source: Photo Network

When we screw on the bottle cap, the threads of the bottle cap and the bottle mouth will cooperate with each other, so that the bottle mouth gradually goes deeper into the bottle cap. After tightening the bottle cap, a certain contact pressure will be generated between the threads, so that the bottle mouth and the sealing gasket fit tightly.

Cut open a bottle cap and see what's inside. Image source: WeChat public account @科普青岛

The mutual compression between the bottle mouth and the bottle cap and the structural design of the connecting bridge ensure the good sealing performance of the bottle cap, but it also becomes an obstacle when twisting the bottle cap.

02How difficult is it for a girl to unscrew a bottle cap?

To unscrew the bottle cap, you first need to overcome the friction between these squeezed surfaces . The greater the contact pressure between the bottle mouth and the bottle cap, the greater the friction between them. Second, you need to apply additional force to break all the connecting bridges .

Image source: Photo Network

In order to measure the difficulty of unscrewing the bottle cap, the industry uses an indicator called loosening torque . In layman's terms: torque is equal to the force applied by the finger on the tangential direction of the bottle cap multiplied by the diameter of the bottle cap. Each beverage bottle cap has its own loosening torque. When the torque applied by the outside is greater than the loosening torque, the bottle cap can be unscrewed .

This diagram can be used to simply understand the concept of torque. F is the force applied by the finger on the tangential direction of the bottle cap, and d is the diameter of the bottle cap.

Source: Drawn by the author

Take Coca-Cola as an example. Its loosening torque is between 1.09 and 1.31 N*m. Studies have shown that nearly half of women are unable to unscrew the cap of a Coca-Cola bottle[1].

Loosening torque diagram of different beverage bottle caps Source: Reference [1]

In addition to the lack of palm strength, pain is also the reason why some girls cannot open the bottle cap. There are vertical stripes on the outside of the bottle cap, which are called anti-slip stripes. They can increase the friction between the fingers and the bottle cap to prevent the fingers from slipping when exerting force, but these uneven stripes will cause pain when the fingers twist the bottle cap.

Image source: Photo Network

Studies have found that women have twice as many nerve fibers per square centimeter of skin as men, so compared to men, women experience more pain when unscrewing bottle caps [2]. In addition, many women often apply hand cream, which reduces the stratum corneum on their skin, and their smooth fingers can easily slip on bottle caps, further increasing the difficulty of unscrewing bottle caps.

03The Secret of Chopping Watermelon with Bare Hands

Splitting a watermelon with bare hands sounds like splitting a brick or opening a durian with bare hands, both of which are macho behaviors. Why can some girls who can't even unscrew a bottle cap split a watermelon with bare hands? This is actually related to the growth characteristics of watermelons. During the ripening process of watermelons, the flesh expands much faster than the rind, which causes the rind to be supported by the flesh and the flesh to be bound by the rind; there is tension in the rind and pressure in the flesh. Even if a ripe watermelon is not subjected to any external force, there is a self-balance of tension and pressure inside it. This phenomenon is called internal stress balance.

There is an internal stress balance in the watermelon. The rind is supported and tensile, while the flesh is bound and compressed.

Source: Drawn by the author

Some varieties of watermelons will have particularly large internal stress when they mature, just like a balloon filled with air, on the verge of bursting. If a little external force is applied at this time, such as a slap or a flick of the finger, small cracks that are difficult to detect with the naked eye will appear in the watermelon rind. The small cracks will destroy the balance of internal stress , just like a balloon being poked by a needle. These small cracks will quickly expand and develop into large cracks that penetrate the rind, causing the watermelon to explode.

You can use a needle to poke a balloon to compare the process of opening a watermelon with your bare hands. Image source: bilibili@The_Maker

This process does not require much force to trigger, so even a girl with little strength can break the watermelon. Some very ripe watermelons even exploded in the watermelon field because of excessive internal stress.

This kind of watermelon that explodes with a single bullet is simply the best among watermelons. Image source: bilibili@Origin Planet

Of course, not all watermelons are so easy to crack open with bare hands. Watermelons that crack easily when touched are generally thin-skinned, juicy, and full-fleshed, which can be explained by Pascal's law . Pascal's law states that the pressure applied to one point in a confined liquid can be transmitted to all directions of the liquid without changing its magnitude. In other words, if the inside of the watermelon is full of water, the pressure generated by hitting the local area can be quickly transmitted to the entire watermelon , and then find the most vulnerable part of the watermelon, forming a crack there. If the watermelon does not have much water and has a cavity inside, such as some sandy melons, the transmission of pressure may be weakened or even blocked, making it not easy to crack open with bare hands.

04If you can’t unscrew the bottle cap, you can do this

Finally, here are some tips for twisting the bottle cap. First, you can twist it through your clothes . This can increase the friction between your hand and the bottle cap to prevent slipping, and can also avoid direct contact between your fingers and the anti-slip pattern on the bottle cap, reducing pain. Secondly, for carbonated beverages like cola, the gas in the bottle will push against the bottle cap, making the squeeze between the bottle cap and the bottle mouth more severe, so you need to overcome greater friction when twisting it. You can give the bottle cap a little downward pressure , and then it will be easier to twist. If you want to open a can in a glass bottle, you can also pat the bottom of the bottle before twisting the lid.

Of course, if you are a martial arts genius, you can also use an elegant roundhouse kick to open the bottle cap.

References

[1] Wang Beihai, Weng Chunmeng. Study on the ability of adult women to open screw caps[J]. Packaging Engineering, 2008, (07):102-103+106.

[2] Tencent Medical Dictionary, 2023, 8 major differences between male and female bodies. I was actually a little moved after reading this...

Planning & Editing: Ding Dong