这是一个力学常识，当你翻转手掌时，手掌是以掌心为中心旋转地。
It's all about rotational mechanics. When you turn your hands over you're rotating them around their centre point, which is near the middle of your palm.
对常识知识推理和这种推理所需实践知识的研究已是并将在一段时间内仍会是人工智能研究的中心问题。
Studies of commonsense reasoning and the practical knowledge required for such reasoning have been and remain the central problem in artificial intelligence.
这篇日志向读者介绍了美国疾控中心长期提倡的应急常识，比如家中准备一个应急包，里面装上水、食物等。
The blog points readers to emergency preparedness tips that the CDC has long advocated, such as having an emergency kit at one's home containing water, food.
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