Mad cow:

The lady reporter:  I am here to collect information on the possible sources of Mad Cow Disease.  Can you offer any reason for this disease? 
    The farmer stared at the reporter and said…    Do you know that a bull mounts a cow only once a year?
Reporter: (obviously embarrassed): Well, sir, that's a new piece of information but what's the relation between this phenomenon and Mad Cow disease?
    Farmer: And, madam, do you know that we milk a cow twice a day?
Reporter: Sir, this is really valuable information, but what about getting to the point?
   Farmer:  I am getting to the point, madam.  Just imagine, if I was playing with your tits twice a day.... and only screwing you once a year, wouldn't you get mad?