Fingerprints; invisible ink; writing on one skin; x-rays.
Introduction to Combing or Brushing One's Hair on Shabbos;
Is One Allowed to Clean a Room in a Shul After a Kiddush or שלום זכר (Shalom Zachar), Etc., Even Though Nobody's Going Back...
Is one allowed to dress a child in their pajamas on Shabbos even though they are not going to sleep until after Shabbos;