Old Mrs Jackson goes down to her doctor's surgery to try and get a Viagra prescription for her ageing husband.
Doctor agrees to writing him a subscription but first he is duty bound to explain about Viagra.
"Well Mrs Jackson, it comes in three strengths... 25% is ok, 50% is pretty good and the 100% strength... well you won't be getting a wink of sleep I promise!"
Mrs Jackson thinks quietly to herself for a while "I'll take the 25% please Doctor.