你的手指正放在键盘或者鼠标上。
当一个事件发生时(比如单击或者鼠标划过)，捕获阶段就开始了。
When an event occurs (a click or a mouse-over, for example) the capture phase begins.
当键盘或者鼠标想发送数据时，它首先必须检查时钟线，确认它处于高电平。
When the keyboard or mouse wants to send information, it first checks the Clock line to make sure it's at a high logic level.
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