Geiger Counter

Source - XKCD
rollover text: At first I didn't get why they were warning me about all those birds sitting on the wire, but then I understood.

See it's funny because Geiger counters click when they detect radiation...and to say 'something clicked' also means that the speaker finally understood what was being said or observed. (source)

So the speaker could mean either that the Geiger counter clicked - suggesting that there was radiation around to be detected and justifying the presence of the Geiger counter itself - or that Cueball finally had his Eureka moment and understood why 'they' wanted him to carry a Geiger counter.

Either way, it's funny because it subverts expectations.