Thousands of Incendiary bombs fell on London during the Blitz (7 Sept 1940 – 11 May 1941) of WW2 and sometimes you can still see them or the remains of them on the Thames foreshore.
Incendiary bombs were often packed into a larger bomb that was released from a plane, with the casing of the bomb opening at altitude, scattering the bomblets in order to cover a wide area. The bomblets would fall to the ground and often land on rooftops, spark and ignite the incendiary material, starting a raging fire.
So many fell on London, that you sometimes see them on the foreshore and can often look like small bits of scaffolding. Fire watchers in London were normally just given a bucket of sand to empty onto the incendiary bombs to stop it sparking if they saw one.