Why do some people let their little fingernail grow long?

On the subway, in a meeting, or sitting at a café terrace, it sometimes happens that a small detail catches the eye without us really knowing why.

A pinky fingernail, longer than all the others. Neither fully flaunted nor truly discreet, it intrigues, raises questions, sometimes even amuses. Is it a matter of vanity, a quirky habit, or a hidden message?