The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning(旋转) of the earth. At the equator(赤道) day and night are sometimes the same length. They are each twelve hours long. The sun rises at 6 in the morning and sets at 6 in the evening. For six months the North Pole is tilted(倾斜) toward the sun. In those months the Northern Hemisphere(半球) gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than night. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    Almost all animals need to sleep to stay healthy. But sleeping can also be dangerous because other animals may hurt those who are sleeping. This leads to many different ways of sleeping that help them stay safe. Some of these ways may surprise you!

    Almost all bats sleep upside down. They hang from a branch of a tree or the ceiling of a cave. Up high, bats are safe from most other animals. If they are in danger, they just fly away.

    A dolphin sleeps with just half its brain at a time. It must stay awake to breathe and to watch for danger. Dolphin mothers and babies can go without sleep for as long as two months to stay safe.

    Some ducks sleep with just half their brains, too. These ducks often sleep in a line. The ducks at both ends help to keep the group safe. They keep one eye open to watch for danger. The ducks in the middle sleep deeply with both eyes closed.

    Seals breathe air, but that doesn't stop some of them from sleeping underwater. They hold their breath and dive deep to sleep for up to twenty minutes at a time. Sleeping deep underwater helps these seals stay safe from animals that hunt them near the surface.

    Horses often sleep standing up. Their legs lock in place so they don't fall over. If a horse is lying down, it needs a long time to get up. A horse can run from danger more quickly if it is already standing.

    A flamingo can sleep while standing on one leg. The standing leg locks so it stays straight. The other leg remains close to the flamingo's body. Its head rests on its back. A flamingo lives (and sleeps) near many other birds to stay safe.

    Some seabirds can sleep while they fly. These large birds spend most of their lives over the ocean. They are in danger on the water, but they are safe in the air. They fly for many hours without flapping（拍打）their wings. Sometimes they sleep while they are flying.

    Would you like to sleep how they sleep?

    Could smells affect your lifespan? Female fruit flies (果蝇) without the ability to smell food live longer than other fruit flies．The sense of smell may be connected to the cellular ageing (细胞老化) process in many other living things  even people．Such connection has recently been found in both worms and flies．For example, scientists found that removing fruit flies' ability to smell made them live nearly 20 percent longer than flies with a sense of smell．

    They confirmed that food smells were the ones most likely to affect ageing, as nutrition (营养) and lifespan are known to be connected in many living things．To test the idea that food smells affect lifespan, they removed flies' ability to smell carbon dioxide, which is produced by some fly foods．They left the rest of the smelling system．

    This action had no effect on male flies, but the females lived 30 percent longer than normal．Removing the ability to smell CO₂ may stop flies getting information about food availability．This could mean that food is not enough, causing processes that encourage flies to survive．Scientists found that the female flies from stored extra fat and that both males and females immune (免疫) to CO₂ were stronger against the stress than normal flies．It is suggested that females may simply be easily influenced by the smells of CO₂ ．

    Scientists point out that people don't know whether ageing and smell are connected in animals and humans．But they clearly experience physical changes after smelling food．

    A "talking kitchen" teaches students how to cook French and speak French. Researchers at Newcastle University in the United Kingdom have developed the French Digital(数字的) Kitchen. Professor Paul Seedhouse led the project.

    "It works like a satellite navigation(导航) system in a car. The system can tell whether you've done what you were asked to do or not. For example, the sensor(传感器) in the knife not only knows that the knife is moving, but it also knows how the knife is moving. So it can discover whether the knife is slicing (切), whether it's scraping (刮) or what. And so it doesn't go on to the next stage of the program unless you've done what it senses(感到)you should have done," said Paul Seedhouse.

    Students can ask the computer to repeat the instructions or translate them into English. There are vocabulary(词汇) lessons before and after the cooking.

    Professor Seedhouse became interested in the idea after he visited a talking kitchen designed for a different purpose. In his opinion the French Digital Kitchen turns the process (过程) of learning language into a real life experience. "Here you're taking it out of the classroom and you're actually using the language to produce something which you can eat at the end of it. It's very enjoyable," he said.

    The system will be available for sale by the end of 2020. Adding the technology to a new kitchen could add about ten to twenty percent to the building costs. The system could also be added to an existing(现存的) kitchen.

    The researchers are also developing a new digital(的呼吸法) kitchen system that is easy to carry or to move. And the European Union has given them money to begin programs in six other languages, including English, Italian and Spanish.