How old is your bb ar? My daughter started to bite her finger nail (note: finger nail not finger bor) when she's 3.5 yrs old and now she's 5. I've been trying many many different method + teacher's help, still no use. So I very very understood what you've been feeling and I could agree no more what you mentioned in your pm:
"自問睇唔算少育兒資訊, 很多 "惡習" 都知道不應讓BB "養成", 樣樣都盡量跟足書教去做.以前自己未有BB, 見到別人的BB呢樣果樣, 一樣會問佢阿媽 (問出口或在心裏問): 點解妳會比佢搞成咁?
而家才知道, 以前太天真無知,以為所有BB都是可以完全由父母PROGRAMME佢的. 當然管教對改造孩子行為是有一定的果效, 但原來總有一些地方是不能到位的."
She got use to hold the small handkerchief when she's born and I recommend her to quit when she's out. During that time, she shifted her security from holding the handkerchief to biting her finger nail. Of course, I won't encourage her to hold her handkerchief again when she's out (but she still needs to touch the handkerchief when she's sleeping).
Now, I decided not to do anything and just wait until she quits biting herself later. If you have any further suggestion, please tell and I definitely would try. This problem troubled us more than a year la.
Mimia
According to a baby/kid development professional, 食手指 is a natural desire. This need has to be fulfilled before he/she will quit naturally. If every time this action is stopped by adult, it will take longer time to fulfill the need then quit. All in all, '食夠就會唔食','越早食夠越早唔食'.