They did all the usual spy stuff like suicide pills, miniature guns and concealed transmitters, but they also ventured into more adventurous territory. The unsuspecting cyclist would then use the pump to inflate the tyres, with fatal consequences. There were also more subtle devices, such as sandals which left authentic native footprints, for use in the Pacific conflict. But perhaps the most twisted genius-spawned, devilishly cunning, steal a line from Blackadder and claim it as your own for additional cunningness, type plan has to have been the Exploding Rat. The concept was simple.
Rats are three-dimensional objects.
And since they are small, fairly common and don't display much regional variation, are easily deployed. The plan was this - hollow out the corpses of rats and fill them with plastic explosives, and all the other stuff necessary to move the plastic explosive from being plastic to being an explosion. The rats would then be dropped into piles of coal near boilers, fires and furnaces, where they would be noticed and thrown into the flames, exploding and causing damage to whatever was nearby. Unfortunately, the concept didn't proceed quite as planned - a shipment of exploding rats was found before they could be used, which meant they weren't used to blow anything up, but gave the German military huge cause for concern and a distrust of rats. Happy research, Kenny J Mobile Rats.