Prince Charles is being shown round a mental hospital. After being shown round the wards and introduced to patients, he is shown into the kitchen for refreshments. He looks up at the ceiling where a man is hanging down, gibbering away to himself.
"What's that man doing up there ?" asks HRH.
"Ah, he thinks he's a light-bulb sir" replies one of the cooks.
"So, why don't you take him down, then ?" asks Charles.
Spoiler:
"What, and work in the dark ?"
Of course it'll fit; you just need a bigger hammer.