For as long as I can remember Matilda has done this thing where she seeks out one of my fingers, preferably the one with the raggediest nail edge, to maniacally stroke.
It soothes and calms her, especially in the middle of the night when she joins us in our bed. If I have my back to her and don't give her my hand she sobs like I've hurt her feelings. On insane, sleepless nights when she has tossed and turned unable to sleep for a variety of reasons, I have let her flick and scratch at my nail till the skin around it has bled.
Parents will do anything for sleep.
Her preference has mostly been the thumb.
(the thumb stroke in action today before swimming lessons... )
No-one else's thumb nail can do the trick. It must be mine. As insane as it can (often!) drive me, it makes my heart feel warm and aglow.