Why do teeth fall out，and why don't they grow back in grown­ups?

　　Baby (or “milk”) teeth do not last long；they fall out to make room for bigger，stronger adult teeth later on.Adult teeth fall out when they become damaged，decayed and infected by bacteria.Once this second set of teeth has grown in，you're done.When they're gone，they're gone.This is because nature figures you're set for life，and what controls regrowth of your teeth switches off.

　　Do old people shrink as they age?

　　Yes and no.Many people do get shorter as they age.But，when they do，it isn't because they're shrinking all over.They simply lose height as their spine(脊柱) becomes shorter and more curved due to disuse and the effects of gravity(重力)．Many (but not all) men and women do lose height as they get older.Men lose an average of 3­4 cm in height as they age，while women may lose 5 cm or more.If you live to be 200 years old，would you keep shrinking till you were，like 60 cm tall，like a little boy again？No，because old people don't really shrink！It is not that they are growing backwards—their legs，arms and backbones getting shorter.When they do get shorter，it's because the spine has shortened a little.Or，more often，become more bent and curved.

　　Why does spinning make you dizzy(眩晕的)?

　　Because your brain gets confused between what you're seeing and what you're feeling.The brain senses that you're spinning using special gravity­and­motion­sensing organs in your inner ear，which work together with your eyes to keep your vision and balance stable.But，when you suddenly stop spinning the system goes out of control，and your brain thinks you're moving while you're not!

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