"on the farm" joke

On a farm out in the country, lived a man and a woman and their
three sons. Early one morning, the woman awoke. While looking out the window toward the pasture, she saw that the family's only cow was lying dead in the field. The situation looked hopeless to her -- how could she possibly continue to feed her family now? In a depressed state of mind, she hung herself.
When the man awoke to find his wife dead, as well as the cow, he too began to see the hopelessness of the situation, and he shot himself in the head.
The oldest son woke to discover his parents dead (and the cow!), and he decided to go down to the river and drown himself. When he reached the river, he discovered a mermaid sitting on the bank. She said, "I've seen all and know the reason for your despair. But if you will have sex with me five times in a row, then I will restore your parents and the cow to you." The son agreed to try, but after four
times, he was simply unable to get it up again. So the mermaid drowned him in the river. The second oldest son woke up. After discovering what had happened, he too decided to throw himself into the river. The mermaid said to him, "If you will have sex with me ten times in a row, then I will make everything right." And while the son tried his best (seven times!), it was not enough to satisfy the mermaid, so she drowned him in the river.
The youngest son woke up, saw his parents dead, the dead cow in the field, and his brothers gone. He decided that life was a hopeless prospect, and he went down to the river to throw himself in. And there he also met the mermaid. "I have seen all that has happened, and I can make everything right if you will only have sex with me fifteen times in a row."
The young son replied, "Is that all? Why not twenty times in a
row?" The mermaid was somewhat taken aback by this request. Then he said, "Hell, why not twenty-five times in a row?" And evenas she was reluctantly agreeing to his request, he said, "Why not THIRTY times in a row?"
Finally, she said, "Enough!! Okay, if you will have sex with me
thirty times in a row, then I will bring everybody back to perfect
health." Then the young son asked, "Wait! How do I know that thirty times in a row won't kill you like it did the cow?