n.脆弱；不坚定
When one or more nodes fail, the downriver nodes can't receive the packet, which makes the system frailness. 树中的节点失效后其下游的节点将不能接收到分组，使得系统变得脆弱。
Psychologists say someone who never says no shows his phenomenon of psychological frailness in interpersonal interaction, which is an immature behavior. 心理学家说，不会说不，这是人际交往中心理脆弱的表现，是不成熟的行为。
