Touchpaper is the little twist of treated paper sticking out of a firework. You put a flame to it, it burns slowly for a few seconds, and that delay is the whole point: it gives you time to walk away before the firework goes off. It is a British word, tied to Bonfire Night, and the instruction printed on every box is famous — 'light the touchpaper and retire immediately'. Because of that, the phrase 'light the touchpaper' is used far more often as an image than as a real action. It means to say or do the one small thing that sets off a huge reaction: 'His comment about the manager lit the touchpaper.' The picture is exact — a tiny act, a short pause, then an explosion you cannot stop. Fireworks now usually come with a fuse instead, and younger speakers may know the phrase without ever having seen the object. You will also see it written 'touch paper' as two words.

（煙火的）引信紙、導火紙。台灣一般統稱「引信」或「導火線」；light the touchpaper 就是「點燃導火線」，比喻引爆事端。