Why Half an Orange?

I once asked comedy legend Barry Cryer to tell me his favourite joke. He told me this one:

“A man walks into a bar and his head is half human and half an orange. The barman asks him how it happened. The man says he rubbed a magic lamp and a genie granted him three wishes. He wished for a big house with a huge swimming pool. Second, the man wished for a loyal, loving wife and the genie granted him his second wish. “What was your third wish?” asks the barman. The man says, “I wished that my head was was half an orange.”