Are the ripples in my thin 3D prints caused by harmonics in my setup, or is there another explanation for the pattern?

Demonstration: putting someone in a room and showing them how to disarm a bomb

Monstration: putting someone in a room and telling them to disarm a bomb

I suspect my dentist's drill can't hurt my tongue, but I'm afraid to test it.

Inspecting things in the dark, beside a light switch.