It's mock aggression in the sense that when people laugh they're often teasing, kidding around.

我们知道打闹是装出来的，因为人一笑,我们就知道他们是闹着玩的。

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They may be knocking down one or two themselves there, because nobody's paying any attention.

他们中的一两个可能会相互打闹或者摔伤,因为没人照看

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They're all over each other sometimes and they can spread saliva or other fluids.

有时候他们会在一起追跑打闹,病毒和疫苗可以通过唾液或其它体液传播

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The general story of why we can't in general is because there's no surprise, there's no mock aggression, and also there may be a general deadening of self-stimulation.

为什么大部分人不能胳肢自己,是因为它在我们意料之内，没有打闹的侵略性，通常这种引自己笑的念头都会嘎然而止。

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They're throwing out insults, maybe they're tickling each other, maybe they're making fun of each other, maybe they're making fun of themselves, and there's some aggression to it.

他们会说伤人的话，胳肢别人，取笑对方，取笑自己，而这些打闹是有一定的侵略性的。

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