Fingerprints; invisible ink; writing on one skin; x-rays.
דבר דבר (Dabair Davar): Addition 1 to 1538: Requests in Davening on Shabbos;
The Halachas of Climbing on a Tree on Shabbos;
using Suspenders, Bending a Spoon or Fork Back into Shape, Detaching Two Tissues, Bending Back the Frame of Eyeglasses, and Putting Foam Stuffing in...