要理解它的真实性，想一想某种简单产品的包装，比如牙膏。
To understand why this is true, think of the packaging commonly used for a simple product, such as toothpaste.
增白牙膏将可以去除牙齿表面色斑，比如由吸烟，喝茶或咖啡引起来的。
Whitening toothpastes will remove only surface stains, such as those caused by smoking, tea or coffee.
比如说，我们的防龋齿牙膏有保质期，那是因为时间长了，里面氟的浓度就降低了。
For instance, our anticavity toothpaste has expiration dating because over time, the fluoride becomes less available to your teeth.
比如说，我们的防龋齿牙膏有保质期，那是因为时间长了，里面氟的浓度就降低了。
For instance, our anticavity toothpaste has expiration dating because over time, the fluoride becomes less available to your teeth.
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