I've slept in a hotel last night, and they had a strange bedside lamp.
It did not have a power switch or a power button. Instead, tapping once anywhere on it's metallic shell would turn it on, and tapping thrice would turn it off.
Being curious, I did some experimentation and discovered thus:
It has to be direct skin-lamp contact, or skin-water-lamp. Any material other than water would block it. I tested keratin (fingernail), thin plastic cloth (t-shirt), slightly thicker cotton (bedsheet), and steel (keyring).
It would also not trigger if it was carefully touched, rather than tapped. It would not register a tap if I was already touching it, and two taps right after each other would register as only one tap.
The lamp did not utilize the plug's grounding peg. So I am guessing that it used it's lack of grounding for trigger. It would run minimal DC voltage (perhaps 1V) to it's shell at miniscule current, and measures rapid changes in this voltage.
If a human (it should work with any creature above certain mass with somewhat conductive skin) touches it, this immediately discharges into them, which triggers the lamp. This would also explain why a tap would not be registered if you are already touching it, and why two taps right after each other would not register separately (no time to charge). But it would not explain why it did not work through metal.
It's really a WTF just because it is a lamp that requires skin contact to function.