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Why Are the Days Longer in Summer?

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WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Did you know that when it’s summer in North America it is winter in some other continents, like South America and Australia? Picture Earth as a round ball with a line drawn around the middle. That line is the equator (ee-QUAY-ter), and it divides our planet into two halves: the Northern Hemisphere (HEM-iss-feer) and the Southern Hemisphere. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. In summer the days are longer, while in winter they are shorter.

On maps and globes, the Earth looks straight up and down, with the North Pole at the top and the South Pole at the bottom. Actually, though, the Earth is tilted 23.4 degrees! (A circle is 360 degrees.) This tilt is the reason that days are longer in the summer and shorter in the winter. The hemisphere that’s tilted closest to the Sun has the longest, brightest days because it gets more direct light from the Sun’s rays. One hemisphere doesn’t stay tilted closer to the Sun all year, however.

Because Earth doesn’t stay in one place! It orbits (or circles) the Sun, making an oval shape. One orbit takes one year. Because the Earth is tilted, one hemisphere will be closest to the Sun for part of the year. But as Earth continues orbiting the Sun, that tilt puts the other hemisphere closer to the Sun. In the Northern Hemisphere, the longest day of the year, called the summer solstice (SOAL-stiss), happens around June 21, because that’s when the North Pole is tilted closest to the Sun. The opposite happens around December 21: the South Pole is closest to the Sun, so the Northern Hemisphere has its shortest day of the year. Meanwhile, the Southern Hemisphere is having its own summer solstice.

Want to see it for yourself? Have an adult help you set up this fun experiment! You can also watch Bill Nye the Science Guy explain the seasons.


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Why Do I Get Cavities?
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WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your mouth is a busy place. There are hundreds of different kinds of bacteria living on your teeth, tongue and gums. Bacteria, as you may know, are tiny little critters that can only be seen with a microscope. We need some of them to help keep us healthy. Others are more of a pain—especially when they cause cavities (CAA-vih-teez), or little holes in our teeth. If you get a cavity, a dentist will have to treat it. You may get a filling.

During the day, you put all kinds of food into your mouth. When you eat, the bacteria that live in there eat, too. Some bacteria make acid out of the sugar in food. So when you eat sweet stuff—candy, cookies, and sugary drinks—you are helping bacteria to make more acid. The acid eats through the enamel, or the hard coating on your teeth. Over time, this can cause cavities and infections. Ouch! Luckily your spit, or saliva (suh-LIE-vuh), has minerals in it, like calcium and phosphate, that fight back by making your tooth enamel stronger and cutting down on acid. The fluoride in toothpaste helps do this, too. If you have a cavity, a dentist can fill the hole to keep bacteria from getting in and causing a bad infection. Unfortunately, there’s another reason for cavities that you don’t control: genes (jeenz).

Genes are little codes inside our cells that get passed down from our parents. They are instructions to our bodies that affect how we look, act, and grow. Genes may play a big part in whether we get cavities. Some people end up with cavities because of their genes, even if they take really good care of their teeth. Other people hardly get any. We can’t change our genes, but we can control other things. Brushing your teeth, visiting the dentist, and avoiding sweets can all help you keep cavities away.

Want to find out more about what lives inside your mouth? Watch the cartoon below from the National Institutes of Health.

 

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Why Do Lizards Lose Their Tails and Regrow New Ones?
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WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Many species of lizard lose their tails when they are being chased. When a predator (PREH-duh-tor)—an animal that wants to eat them, like a bird or cat—grabs for the tail, it falls off, and the lizard can run away. How does it happen? Scientists looking at geckos (a type of lizard) under a microscope have found out that some parts of the tail are weaker than other parts.

It’s no accident. The sections of a lizard’s tail will hold together until the lizard is in danger. Scientists think that’s when special cells inside the tail make chemicals that attack the weak points. Then—surprise!—the tail breaks off. It will also keep wiggling for a while to distract the predator while the lizard speeds off. Pretty awesome! That’s not the end of the story, either. Most lizard species can also grow a new tail. 

Lizard tails have a lot of jobs to do. Lizards move their tails around to “talk” to each other. It’s a kind of secret lizard language. The tail also helps lizards keep their balance when they’re scrambling up a tree or jumping from rock to rock. So even though losing the tail is a useful trick, growing it back is pretty important, too. Sometimes lizards can get carried away, though: one busy lizard from Argentina grew six new tails at once!

Watch a gecko regrow its tail in this video

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