Incisors門牙
8 in both the baby set and the adult set. They are usually the first teeth to fall out, around age 6 or 7.
乳牙和恆牙都是 8 顆；通常最先掉，大約 6、7 歲。
The Human Body · Lesson 11 · 人體探索 第11課
Why Do We Get Two Sets of Teeth?為什麼會換牙？
Baby teeth hold the place while adult teeth grow inside your jaw. Slide from age 3 to 20 and watch them trade places, then count your own teeth.
乳牙先占位置，恆牙在頷骨裡慢慢長。把年齡從 3 歲拉到 20 歲，看它們換班，再數數你自己的牙。
Big idea · 一句話看懂
Baby teeth hold the place while adult teeth grow inside your jaw. From about age 6 to 12, twenty baby teeth fall out and up to thirty-two adult teeth take over, and you only get one adult set.
乳牙先占位置，恆牙在頷骨裡長大。大約 6 到 12 歲，20 顆乳牙一顆顆掉，最多 32 顆恆牙接班，而恆牙只有這一套。
3D Model · 3D 模型
Watch Your Teeth Trade Places 看牙齒換班
Slide the age from 3 to 20. White baby teeth fall out one by one, while ivory adult teeth climb up out of the jaw bone to take their place, and the big molars arrive at the back. Tap any tooth to see its name and when it usually comes in. Then use the tooth chart below the model to count your own teeth.
把年齡從 3 歲拉到 20 歲：白色的乳牙一顆顆掉落，象牙色的恆牙從頷骨裡往上長出來接班，後面的大臼齒也一顆顆報到。點任何一顆牙，看它的名字和大約幾歲長出來。再用模型下方的牙齒圖，數數你自己的牙。
Loading… · 載入中…
Drag to turn · 拖曳旋轉 Tap a tooth · 點一顆牙 Scroll or pinch to zoom · 滾輪／雙指縮放
This 3D model needs WebGL, which this browser does not support. The tooth chart and the cards below still work.
這個瀏覽器不支援 WebGL，無法顯示 3D 模型；下方的牙齒圖和卡片一樣能用。
My tooth chart · 我的牙齒圖
Your left · 你的左邊as you see it in a mirror · 照鏡子看到的樣子Your right · 你的右邊
Baby · 乳牙Adult · 恆牙Gap · 空位1–2 incisors · 門牙 3 canine · 犬齒 4–5 premolars or baby molars · 小臼齒或乳臼齒 6–7 molars · 大臼齒
How a cavity forms · 蛀牙怎麼形成
- Bright white: baby teeth
亮白色：乳牙 - Ivory: adult teeth
象牙色：恆牙 - Pale: adult teeth still waiting inside the bone
淡淡的：還在骨頭裡等待的恆牙 - Gold: the tooth you picked
金色：你點選的牙
The skull and jaws are real, from the same BodyParts3D skeleton as Lesson 1, which has no teeth; the teeth are our own drawing, at about life size. Every child is different: the ages in the model are typical averages from dental charts, and a year or two early or late is normal. The jaw itself also grows a lot during childhood, which the model does not show.
頭骨和上下頷骨是真實的，和第一課同一副 BodyParts3D 骨架（資料裡沒有牙齒）；牙齒是本站自繪，大約是真實大小。每個孩子都不一樣：模型用的是牙科資料上的平均年齡，早一兩年或晚一兩年都很正常。真實的頷骨在成長過程中也會長大很多，模型沒有畫出來。
Bone models: BodyParts3D, © The Database Center for Life Science, licensed under CC BY 4.0. We selected the bones, simplified the shapes, and turned the model upright; the teeth are our own drawing. · 骨骼模型來源：BodyParts3D（日本生命科學資料庫中心，CC BY 4.0 授權）；本站挑選骨頭、簡化形狀並轉正方向，牙齒為本站自繪示意。
Reading · 英文閱讀
11
Unit 11: Why Do We Get Two Sets of Teeth?
Smile in a mirror and count your teeth. Most children have all 20 of their baby teeth by about age 3. Starting around age 6, the baby teeth fall out one by one, and adult teeth take their place. By about age 12 or 13, most children have lost all of their baby teeth. An adult can have up to 32 teeth, but there is no third set. Adult teeth have to last for the rest of your life.
對著鏡子笑一笑，數數你的牙齒。大多數孩子大約 3 歲時就長齊 20 顆乳牙。從 6 歲左右開始，乳牙一顆顆掉落，恆牙接著長出來。大約 12、13 歲時，多數孩子的乳牙都換完了。大人最多可以有 32 顆牙，但是沒有第三套：恆牙要用一輩子。
Baby teeth are not just for practice. They help you chew and speak clearly, and they hold space in your jaw for the adult teeth. The adult teeth have been growing inside your jaw bone for years, right under the baby teeth. As an adult tooth moves up, the roots of the baby tooth above it slowly melt away. That is why a loose baby tooth has almost no root when it finally falls out.
乳牙可不只是拿來練習的。它們幫你咀嚼、讓你說話清楚，還在頷骨裡幫恆牙保留位置。恆牙早就在頷骨裡長了好幾年，就在乳牙的正下方。恆牙往上移動時，上面那顆乳牙的牙根會慢慢被吸收掉，所以搖搖欲墜的乳牙掉下來時，幾乎已經沒有牙根了。
Different teeth do different jobs. The flat incisors at the front bite and cut, like scissors. The pointed canines next to them grip and tear. The premolars and molars at the back have wide, bumpy tops for grinding food into a mash. Your first adult molars come in around age 6, behind the baby teeth, without pushing any baby tooth out, so many people do not even notice them. They are called six-year molars.
不同的牙齒做不同的工作。前面扁扁的門牙負責咬斷、切開，像剪刀一樣；旁邊尖尖的犬齒負責抓住、撕開；後面的小臼齒和大臼齒頂端寬寬的、凹凸不平，負責把食物磨碎。第一顆恆牙大臼齒大約 6 歲時長在乳牙的後面，不會把任何乳牙推掉，所以很多人根本沒注意到它，大家叫它「六歲臼齒」。
A tooth has layers. The outside is enamel, the hardest substance in your body. Under it is dentin, which is softer and a little yellow, and in the center is the pulp, which has nerves and blood vessels. The roots hold the tooth tightly in the jaw bone. Teeth are hard like bones, but they are not bones: a broken bone can heal, but enamel cannot grow back, so a hole in a tooth stays a hole until a dentist fills it.
牙齒是一層一層的。最外面是琺瑯質，是全身最堅硬的物質；下面是比較軟、帶點黃色的象牙質；中間是牙髓，裡面有神經和血管。牙根把牙齒牢牢固定在頷骨裡。牙齒和骨頭一樣硬，但牙齒不是骨頭：骨頭斷了會癒合，琺瑯質卻長不回來，所以牙齒上的洞會一直是個洞，直到牙醫把它補起來。
Cavities start with bacteria that live on your teeth in a sticky film called plaque. When you eat sugar, the bacteria eat it too and make acid, which slowly dissolves the enamel. If the hole reaches the dentin and then the pulp, the tooth can really hurt. Fluoride helps enamel stay strong and can even repair the very earliest damage, which is why most toothpaste has fluoride in it.
蛀牙是從住在牙齒上的細菌開始的，它們藏在一層黏黏的薄膜裡，叫做牙菌斑。你吃糖的時候，細菌也跟著吃，並且產生酸，酸會慢慢溶解琺瑯質。洞如果深到象牙質、再到牙髓，牙齒就會很痛。氟能讓琺瑯質保持堅固，甚至能修補最早期的損傷，所以大部分的牙膏都含氟。
Brush for two minutes, twice a day, with fluoride toothpaste, and clean between your teeth. In Taiwan, elementary students rinse with a fluoride mouthwash at school once a week, and first and second graders can get sealants: a thin coating that fills the deep grooves of the six-year molars, where food likes to hide. Eat fewer sweets and sugary drinks, and see a dentist every six months. Now count your own teeth with the tooth chart below the model.
每天刷牙兩次、每次兩分鐘，使用含氟牙膏，也要清潔牙縫。在台灣，國小學生每週在學校用含氟漱口水漱口一次，一、二年級的學生還可以做窩溝封填：用一層薄薄的材料填平六歲臼齒上深深的溝，那裡最容易卡食物。少吃甜食、少喝含糖飲料，每六個月看一次牙醫。現在用模型下方的牙齒圖，數數你自己的牙。
Read aloud · 課文朗讀
Questions · 閱讀理解
Why are baby teeth important even though they fall out?
They help you chew and speak clearly, and they hold space in the jaw for the adult teeth.
Why does a loose baby tooth have almost no root?
As the adult tooth underneath moves up, the roots of the baby tooth slowly melt away.
How do cavities form?
Bacteria in plaque eat sugar and make acid, and the acid slowly dissolves the enamel.
Words & Phrases · 生字及片語
tooth(n.)牙齒（複數 teeth）My little sister lost her first tooth today.我妹妹今天掉了第一顆牙。baby tooth(n.)乳牙A baby tooth gets loose before it falls out.乳牙掉下來之前會先鬆動。loose(adj.)鬆動的My front tooth is so loose that it wiggles.我的門牙鬆到會搖來搖去。chew(v.)咀嚼；嚼Chew your food well before you swallow.吞下去之前，要把食物好好嚼碎。cavity(n.)蛀牙；蛀洞The dentist found a small cavity in my molar.牙醫在我的臼齒上發現一個小蛀洞。brush(v.)刷I brush my teeth after breakfast and before bed.我在早餐後和睡前刷牙。enamel(n.)琺瑯質Enamel protects the outside of each tooth.琺瑯質保護每顆牙齒的外層。dentist(n.)牙醫We visit the dentist every six months.我們每六個月去看一次牙醫。
Quick Check · 小測驗
1. How many baby teeth do most children have?
2. What are the flat front teeth that bite and cut called?
3. What is the hardest substance in your body?
4. What do bacteria make from sugar that harms your teeth?
5. According to the reading, what do sealants do?
Kinds of Teeth · 牙齒的種類
Scissors, hooks, and millstones 剪刀、勾子和石磨
A baby set has 8 incisors, 4 canines, and 8 molars: 20 teeth. An adult set adds 8 premolars and 4 more molars, plus up to 4 wisdom teeth: up to 32.
一套乳牙有 8 顆門牙、4 顆犬齒、8 顆臼齒，共 20 顆。恆牙多了 8 顆小臼齒和 4 顆大臼齒，再加上最多 4 顆智齒，最多 32 顆。
Canines犬齒
4 in each set. Their roots are the longest of all your teeth.
每一套 4 顆；它們的牙根是所有牙齒裡最長的。
Premolars小臼齒
Adults only: 8 premolars grow in where the baby molars were, at about ages 10 to 12.
只有恆牙有：8 顆小臼齒大約 10 到 12 歲時，長在原本乳臼齒的位置。
Molars大臼齒
The six-year molars come in behind the baby teeth, and the twelve-year molars come in behind those.
六歲臼齒長在乳牙後面，十二歲臼齒再長在它們後面。
Wisdom teeth智齒
Up to 4, coming in between about 17 and 25, if at all. Some people never get them.
最多 4 顆，大約 17 到 25 歲才長；有些人一輩子都沒有。
Measure Yourself · 親身測量
Count your teeth and find your tooth age 數一數牙齒，算出牙齒年齡
Use a mirror and the tooth chart below the model. Tap each spot to mark a baby tooth, an adult tooth, or a gap, and the chart works out about how old your teeth look.
準備一面鏡子，搭配模型下方的牙齒圖。點每個位置，標出乳牙、恆牙或空位，牙齒圖會算出你的牙齒大約像幾歲。
- 1Look in a mirror · 照鏡子Wash your hands, smile wide, and look at one side at a time. New adult front teeth are bigger and often have little bumps along the edge.先洗手，張大嘴笑，一次看一邊。剛長出來的恆牙門牙比較大，邊緣常常有小小的鋸齒。
- 2Tap the chart · 點牙齒圖Mark each spot as a baby tooth, an adult tooth, or a gap. Six-year molars are the big teeth right behind your baby molars.把每個位置標成乳牙、恆牙或空位。六歲臼齒就是乳臼齒後面那顆大大的牙。
- 🦷Find your tooth age · 算出牙齒年齡The chart compares your mouth with typical ages and shows that age in 3D. Every child is different, so being a year or two early or late is normal.牙齒圖會拿你的嘴巴和一般的年齡比較，並在 3D 模型裡顯示那個年齡。每個孩子都不一樣，早一兩年或晚一兩年都很正常。
Myth vs. Fact · 常見迷思
Four things people get wrong 四個常見的誤會
✗ Myth · 迷思Baby teeth don't matter, because they fall out anyway.乳牙反正會掉，蛀了也沒關係。
✓ Fact · 事實Cavities in baby teeth can hurt and spread. Losing a baby tooth too early can let other teeth drift into the space the adult tooth needs.乳牙蛀了一樣會痛，還可能越來越嚴重。乳牙太早掉，旁邊的牙會移過來，占掉恆牙需要的位置。
✗ Myth · 迷思Sugar itself makes holes in your teeth.糖本身會在牙齒上蛀出洞。
✓ Fact · 事實Bacteria eat the sugar and make acid, and the acid makes the holes. How often you snack matters as much as how much sugar you eat.是細菌吃了糖產生酸，酸才把牙齒蛀出洞。多常吃零食，和吃了多少糖一樣重要。
✗ Myth · 迷思Brushing harder cleans your teeth better.刷牙越用力，牙齒越乾淨。
✓ Fact · 事實Scrubbing hard can hurt your gums and wear down your teeth. Gentle brushing for two minutes does the job.太用力會傷到牙齦、磨損牙齒。輕輕地刷兩分鐘就夠了。
✗ Myth · 迷思Teeth are bones.牙齒是骨頭。
✓ Fact · 事實Teeth and bones are both hard and full of calcium, but teeth are not bones. A bone can heal itself; enamel cannot. That is why teeth are not counted in your 206 bones.牙齒和骨頭都很硬、都有很多鈣，但牙齒不是骨頭：骨頭會自己癒合，琺瑯質不會。所以 206 塊骨頭裡沒有算牙齒。
Remember It · 記憶口訣
Your teeth in a sentence 一句話記住牙齒
6 and 126 歲和 12 歲
Big new molars arrive at the back at about age 6 and again at about age 12.
大約 6 歲和 12 歲時，後面各長出一組新的大臼齒。
Scissors, hooks, millstones剪刀、勾子、石磨
Incisors cut, canines grip, and molars grind.
門牙切、犬齒勾、臼齒磨。
Two, two, two二、二、二
Brush two times a day, for two minutes each time, and see the dentist two times a year.
一天刷兩次、每次刷兩分鐘、一年看兩次牙醫。
Bugs plus sugar make acid細菌＋糖＝酸
No sugar for the bacteria means no acid for your teeth.
不給細菌吃糖，牙齒就不會泡在酸裡。
In Chinese · 中文怎麼說
Up on the Roof, Down Under the Bed 上牙丟床底，下牙丟屋頂
In Taiwan, many families have a custom for a lost baby tooth: a lower tooth is thrown up onto the roof, and an upper tooth goes under the bed, so the new tooth will grow in the right direction, toward where the old one went. In many English-speaking countries, children put the tooth under their pillow, and the Tooth Fairy swaps it for a coin. And in Chinese, a wisdom tooth is 智齒 (zhìchǐ), just like in English: it comes in when you are old enough to be wise.
在台灣，很多家庭對掉下來的乳牙有個習俗：下排的牙丟到屋頂上，上排的牙丟到床底下，希望新牙朝著舊牙去的方向，長得又快又直。在許多說英語的國家，孩子會把牙齒放在枕頭下，牙仙子（Tooth Fairy）會把它換成一枚硬幣。還有，中文的「智齒」和英文的 wisdom tooth 意思一樣：等你長到夠有智慧的年紀，它才會長出來。
Classroom Activity 1 · 課堂活動一
The Eggshell Test 蛋殼實驗
You need · 準備材料
- Three hard-boiled eggs and three clear cups三顆白煮蛋和三個透明杯子
- Water, white vinegar, and cola清水、白醋和可樂
Steps · 步驟
- Put one egg in each cup. Cover one with water, one with vinegar, and one with cola.每個杯子放一顆蛋，分別倒入清水、白醋和可樂，淹過雞蛋。
- Watch the vinegar cup for a few minutes. What do you see on the shell?觀察白醋杯幾分鐘，蛋殼上出現了什麼？
- Leave the cups overnight, then take the eggs out and gently feel and look at each shell.放置一個晚上，再把蛋拿出來，輕輕摸摸看、仔細看看每個蛋殼。
- Try brushing the cola egg with a toothbrush and toothpaste. What comes off, and what does not?用牙刷和牙膏刷刷看可樂杯的蛋，什麼刷得掉、什麼刷不掉？
Teaching tip: eggshell is not enamel, but both are hard minerals made with calcium, and acid dissolves them. The bubbles in the vinegar show the shell breaking down. The cola egg shows both stains and acid. Remind students that teeth have saliva and fluoride to help them, but they still need brushing and fewer sugary drinks.
教學提示：蛋殼不是琺瑯質，但兩者都是含鈣的堅硬礦物，都會被酸溶解。白醋裡冒出的泡泡，就是蛋殼正在分解；可樂杯同時看得到染色和酸的作用。提醒學生：牙齒有口水和氟幫忙，但還是需要刷牙、少喝含糖飲料。
Classroom Activity 2 · 課堂活動二
Our Lost Tooth Graph 全班掉牙統計圖
You need · 準備材料
- Sticky notes or a tally chart on the board便利貼，或黑板上的劃記表
- The 3D model and the tooth chart on this page本頁的 3D 模型和牙齒圖
Steps · 步驟
- Each student counts how many baby teeth they have lost so far.每位同學數一數自己到目前為止掉了幾顆乳牙。
- Make a bar graph: number of lost teeth across the bottom, number of students up the side.畫長條圖：橫軸是掉了幾顆牙，縱軸是有幾位同學。
- Set the 3D model to your class's age. How many baby teeth has it lost?把 3D 模型調到你們班的年齡，模型掉了幾顆乳牙？
- Is everyone the same? What is the most common number?每個人都一樣嗎？最常見的是幾顆？
Teaching tip: the graph usually spreads out widely, which is a good way to show that a typical age is only an average. Be kind to students who have lost many teeth or none at all: both are normal.
教學提示：統計圖通常分布得很開，正好說明「一般的年齡」只是平均值。掉了很多顆或一顆都沒掉的同學都很正常，請溫和地對待。
⚕ Health note · 健康提醒This lesson is for learning about the body. It is not medical advice. If a tooth hurts, your gums bleed or swell, or a tooth gets knocked out, tell an adult right away. A knocked-out adult tooth can sometimes be saved if you see a dentist quickly.本課是認識身體的教材，不是醫療建議。如果牙齒痛、牙齦流血或腫起來，或牙齒被撞掉，請馬上告訴大人。被撞掉的恆牙如果很快去看牙醫，有時還救得回來。