A zoo had among its animals a female gorilla, whose mood was becoming increasingly difficult.
The vet concluded that she was on heat and that a mate should be found.
The vet contacted some other nearby zoos to find a partner for the broody female, but to no avail.
The female gorilla's behaviour continued to worsen, but the vet noticed that she grew calmer, and strangely responsive, whenever a particularly well-built and not-too-handsome zoo keeper entered the enclosure.
Being an unprincipled and adventurous fellow, the vet put an outrageous proposition to the keeper: For a fee of Rs.5000 would the keeper consider spending a little 'quality time' with the gorilla, purely in the interests of research of course?....
The keeper pondered the suggestion, and after a few minutes agreed to the offer, subject to three conditions. The vet, intrigued, listened to the keeper's demands:
"First," the keeper said, "This thing should happen only after zoo hours."
"Fine," said the vet.
"Second, no-one must ever know."
"You have my word," said the vet, "And your final condition?"
"It's just," said the keeper a little awkwardly, "I require at least a month to save Rs.5000, would you mind giving me a credit period?"