For 5V circuit it is no concern I guess, but the moment voltage becomes 12V or higher, it starts to get "hot".
Since when is 12V considered dangerous?
30V is still safe, and 20 years ago people were stronger, 60V was safe then.
I know this guy, he is now 70+. He used to work as an electrician on his own, probably doing a good enough job considering that he was doing it forever.
He does not have any considerable education, he just probably learned on the job.
Well, is not like I would do the same or I want to encourage anyone, but to look for the hot wire this guy used to lick his finger and touch the live copper until he felt it... Here we have 230V 50hz.
Everyone who saw him doing it including myself, many many times, tried to persuade him not to, but he just kept going...
He is still alive though.
That is very dangerous though and no one should do it.