share some scientific knowledge

#What you see is very scientific#

1. Each star has its own unique spectral signature, similar to fingerprints of chemical elements. By analyzing spectrum of a star, astronomers can determine its composition and temperature.

2. The oldest rocks on Earth can be traced back to about 4.5 billion years ago. These rocks are called "Greenland rocks" which are earliest evidence for formation of earth's crust.

3. The fur of polar bears is actually transparent, and their fur helps them store heat and stay warm. When sunlight hits a polar bear's fur, light is refracted by transparent hairs, creating a white appearance.

4. When resistance is heated, its resistance value will increase with temperature rise. This is due to increased collision of electrons in material and increased resistance resistance.

5. In a daily human diet, 1 gram of fat can provide 9 kcal of energy, while 1 gram of carbohydrates and proteins can only provide 4 kcal of energy.

6. The energy in stars comes from nuclear fusion reactions, in which hydrogen nuclei fuse to form helium nuclei, releasing vast amounts of energy. This reaction continues in core of star.

7. The color of snow is determined by its crystal structure, not by color of material itself. The structures of snow crystals reflect and scatter light, making what we see appear white.

8. A black hole is an extremely dense celestial body with such a strong gravitational force that not even light can escape. The boundary of a black hole is called "event horizon", beyond which matter is absorbed by black hole.

9. Shark rays are made up of shark teeth arranged in a vortex in shark's mouth. Shark rays are very tough and are used to protect shark's mouth and teeth.

10. Iron is most abundant metallic element on Earth, making up about 5% of earth's crust. The Earth's inner core is primarily composed of iron and nickel, and high density of these metals allows Earth to exert a strong gravitational pull.

11. Clover is not a specific type of plant, but a variety of it. Most shamrocks have three leaves, but four leaf clover is a very rare variety, with a probability of about 1 in 10,000.

12. Human DNA is very similar to that of other species, up to 98% similar to gorilla DNA, and about 90% similar to cat DNA.

13. The speed of Earth's rotation is not constant, but is gradually slowed down by friction against ground. Every hundred years, Earth's rotation slows down by about 0.0015 seconds.

14. The vast majority of atoms are empty. If nucleus of an atom is enlarged so that its diameter is length of a football field, electrons outside it will float around edge of nucleus, creating an image similar to that of a mosquito flying across a football field. .

15. Venus is hottest planet in solar system with a surface temperature of around 900°F (around 475°C), hotter than boiling water.

16. Albert Einstein's theory of relativity predicted that gravity would affect speed of time. A stronger gravitational field causes time to flow more slowly, which is known as "gravitational time dilation".

17. The sun produces about 7 million tons of energy per second, which is related to energy released from nuclear fusion reactions. This energy is generated in very heart of Sun and then gradually radiates out into outer regions of Sun.

18. While some metals are highly conductive, electrical current in metals typically travels at several inches per hour, so metals conduct electricity relatively slowly.

19. Experiments with short-term collisions have shown that when two steel balls collide at a speed close to speed of light, a more powerful release of energy will be produced than in a nuclear explosion. This is due to fact that closer speed in mass-energy equation E = mc² to speed of light c, more pronounced difference in energy release.

20. Physicists have developed scales for quantitatively measuring force of gravity. This scale is known as "stiffness unit" and can be used to determine weight of an object in a gravitational field by comparing gravitational force between two objects.