Heat Transfer Powered Torch

The problem with torches is that when you need them, they are probably out of battery. There are numerous types of battery-less torch out there which require you to constantly flick/shake/twist/wind them when you need long duration of usage, but I will introduce a better idea: a torch that lights up when you stick the tail part in your mouth (or somewhere else warm, go figure). The difference of temperature between tail and head portion causes heat transfer, and this generates power to light up the torch. An additional advantage is that you got both your hands free. Think of a situation when during a winter night your car broke down in the middle of nowhere. You get the idea.

Oh, an obviously more novel idea is SOLAR-POWERED torch. But someone has already invented it.