A signal is an action that distinguishes one type of person from a would-be mimic because it would be too costly for the mimic to carry out.
信号是区别一类人和一个有意模仿者的行为，因为这个行为代价过高，模仿者很难做到。
2
PROBLEM: Wi-Fi USES frequencies that can't carry a signal more than a few tens of meters.
问题：Wi - Fi使用了传送信号距离不超过几十米的频率。
3
If supervise were to send a signal to kill its child process, the shell would receive the signal and exit, but the JVM would carry on running and become an orphan.