Seen on reddit:
A man sees a new medical clinic has opened on his block. The clinic has a bold sign that says - "$20 per visit, and if we can't cure you, will give you $100". He thinks to himself, easy money. So he walks into the clinic and tells the doctor he's lost his sense of taste. The doctor takes brings in a small vial of medicine and puts one drop on his tounge. The man recoils - "Eww, that's kerosene!" he cries. The Doctor turns to him and says - 'Looks like your sense of taste is back, that'll be $20"
The next week he tries again. This time he claims that he's suffering from memory loss, and pretends he'd never visited. The doctor again takes out a small vial, puts a drop on his tounge, and he says - "Eww, that's the same Kerosene you gave me the last time". "Looks like your memory is back, says the doctor, that'll be $20".
The guy waits two weeks, and tries to get $100 out of the doctor one last time. This time he enters the clinic and claims he is having vision problems. The doctor runs a few tests, and ultimately says to him. "You know what, I don't think I can fix your vision problems. Here's your $100" and hands the man a bill. The man takes the bill to put it in his wallet, and realizes that the doctor handed him a $20 bill! "Hey," he says to the Doctor, " what are you trying to pull? This is a $20!" The Doctor then grabs the $20 out of his hand and says - " well, I guess I did cure your vision problem after all, and you owe me another $20!"