The truest explanation of recycling involves taking a material, melting(融化) it down, and turning it back into itself over and over. This can be done with glass and metal, which can both be remelted and reformed into bottles or cans forever. This is a closed-loop(闭环) system, so it’s very desirable. On the other hand, some materials slowly degrade(降解) over time, meaning they can be reformed maybe once or twice, but after a while the chemical composition of the original substance has changed and it can no longer be turned back into what it once was. This is called downcycling(降级回收).

The downcycling of plastic is one of the serious issues surrounding this material. For example, plastic water or soda bottles are seldom turned back into bottles. The other is the fact that plastic never biodegrades(生物降解). Many materials newspaper included, will biodegrade at the end of their lives. When paper enters the environment, given exposure to the air, it breaks down, leaving the world undamaged. Plastic, on the other hand, photodegrades(光降解), and this is very concerning. As discussed earlier, in the process of photodegrading, wind, sun, and water break plastic down into smaller and smaller particles(颗粒) that will cause great damage to the ocean ecosystem.

Finally, there is concern about heating food in plastic containers. Avoid reheating food in plastic containers in the microwave, and stop covering food with plastic wrap. Storing cool food in plastic containers isn’t as big a concern, but avoid heating food and plastic together to eliminate the possibility of plastic into your food.

So the disadvantages to plastic are that it can’t really be recycled in the true meaning of the term, it will never disappear from our environment because it can’t biodegrade, and it is a vehicle to deliver dangerous chemicals into our food chains.

Teenagers are risk-takers. Those things may not be understood by the adults around them. Are they driven by poor judgment or immature brains?

In their book Wildhood, the bestselling authors Barbara Natterson-Horowitz and Kathryn Bowers，present findings from their five-year study on wild animal adolescence. The two performed a complete survey of age-related death in wild animals and found something truly surprising-young folks are born to take risks, and with good reasons.

Life on Earth for adolescent and young adult animals is just as dangerous. In the wild young animals crash, drown and starve more often than their adult animals. And they are more likely to be targeted and killed by predators. Old enough to be away from home? But new to the ways of the world? This time of life is risky for human adolescents as well. Wildlife biologists describe these unprepared creatures as predator naives.

Facing death danger while still maturing is a fact of life for adolescents across species. Adolescents must have predator awareness if they are to survive. Said another way: To become safe, you must take risks.

Adult bats typically flee predators such as barn owls. But adolescent bats have been seen doing the opposite-flying toward them. This behavior can also be seen in other wild animals. Biologists call it “predator inspection”. Predator inspection provides young animals important information about their deadly predators. Familiarity with predatory strategies helps adolescents survive and even avoid future attacks. Predator inspection is one of the ways to help predator naives to become experienced predators.

Studying wild animal adolescence offers a new understanding of why teenagers act the way they do. Parents should balance the urge to protect with the need to let their teenagers test their wings. The research suggests that adolescents need some exposure to the dangers they will someday face on their own — and without knowing that, parents’ over protection maybe the riskiest behavior of all.

About ten men in every hundred suffer from color blindness in some way. Women are luckier; only about one in two hundred is affected in this matter. Perhaps, after all, it’s safer to be driven by a woman!

There are different forms of color blindness. In some cases, a man may not be able to see deep red. He may think that red ,orange and yellow are all shades of green. Sometimes a person cannot tell the differences between green and blue. In rare cases an unlucky man may see everything in shade of green—a strange world indeed.

Color blindness in human beings is a strange thing to explain. In a single eye there are millions of very small things called “cones”. These help us to see in a bright light and to tell difference between colors. There are also millions of “rods”, but these are used for seeing when it is near dark. They show us shape but no color.

Some insects have favorite colors. Mosquitoes prefer blue to yellow. A red light will not attract insects, but a blue lamp will. In a similar way human beings also have favorite colors. Yet we are lucky. With the aid of cones in our eyes, we can see many beautiful colors by day, and with the aid of rods we can see shapes at night. One day we may even learn more about the invisible colors around us.