Blue touch paper

Blue touch paper is a type of paper fuse for explosives. The second and third incarnations of the Doctor used small amounts of explosive wrapped in blue touch paper to destroy locks. (DW: The War Games, Spearhead from Space)

The fourth incarnation of the Doctor once used a firecracker and blue touch paper to unclog the gas pocket powering the Sacred Flame. He later gave another piece to the Sisterhood of Karn in case it should happen again. When they asked him what the strange symbols meant he told them "light the blue touch paper and stand well back". (DW: The Brain of Morbius)