Guinness

Paddy walks alone into his local of 30 years and goes to the bar.

"Give me 10 pints of Guinness, please Mick"

Mick, thinking there must be a group of Paddy's mates coming duly pulls 10 pints of Guinness, lining them up on the bar. Paddy proceeds to then drink the first, the third, the fifth, the seventh, and the ninth pint of Guinness. He then pays the bill leaving the second, the fourth, the sixth, the eighth, and the tenth pint of Guinness untouched.

Next night in walks Paddy, again ordering 10 pints of Guinness. Once they are all lined up he goes through the same procedure drinking the first, the third, the fifth, the seventh, and the ninth pint of Guinness. He then pays the bill leaving the second, the fourth, the sixth, the eighth, and the tenth pint of Guinness untouched.

The following evening Paddy hits the bar again, ordering ten pints but only drinking the first, the third, the fifth, the seventh, and the ninth pint of Guinness. He then pays the bill leaving the second, the fourth, the sixth, the eighth, and the tenth pint of Guinness untouched. He's about to walk out when Mick stops him and asks;

"Why have you come in here the last three nights, ordered 10 pints of Guiness but only drunk the first, the third, the fifth, the seventh, and the ninth pint of Guinness?"

"Oh Mick, I'll tell you why I only drink the first, the third, the fifth, the seventh, and the ninth pint of Guinness -

...the doctor told me I must only drink the odd pint of Guinness!"