1. Expression：in one’s shell
Explanation：In a quiet or withdrawn state. （处于安静或孤僻的状态）
1. 比喻某个计划，策略等还在酝酿中，尚未确定。2. 指有些人生性害羞，性格内向，不喜欢和人交谈。
Usage：也可以说为：into one’s shell，常和动词go 搭配，即：go into one’s shell.
Example：After getting mugged, I went into my shell and didn't speak to anyone for nearly a week.
I'm not good around large groups of people; I usually just go into my shell and wait until I can be alone.
Her rejection of him seem to have make him go into his shell.
A: My daughter is always in her shell when a stranger is around. I am a bit worried.
B: Take it easy. Just encourage her to make more friends.
2. Expression：out of one’s shell
Explanation：Less shy, reserved, or reticent and in a more sociable, outgoing, or enthusiastic state. （不再拘谨，不再沉默）
Example：Samantha was quite a quiet girl growing up, but she really started coming out of her shell in college.
Jack is actually a very funny, engaging guy once he's out of his shell.
She used to be so quiet, but now she's really coming out of her shell and chatting to everyone.
她一向沉默寡言, 但现在可真活了, 跟谁都能谈得来。
Music drew the shy girl out of her shell.