e-mail i got explaining it :

Once upon a time a man appeared in a village
and announced to the villagers that he would
buy monkeys for $10 each.

The villagers, knowing that there were many monkeys around,
went to the forest and caught some.
The man bought thousands at $10, but, as supply dwindled,
the villagers reduced their efforts.

Then the man announced that he would now buy monkeys at $20 each.
With this incentive, the villagers started catching monkeys again.
Soon the supply diminished even further and people returned to their farms.

The man increased the offer to $25 and the supply of monkeys
became dwindled so low it was an effort to even find a monkey,
let alone catch one!

Then the man announced that he would pay $50 for each monkey!
However, as he had to go to the city on business,
his assistant would buy monkeys on his behalf.
In the absence of the man, the assistant told the villagers:
"Look at all the monkeys in the big cage that the man has already collected.
I will sell them to you at $35 and when the man returns from the city,
you can sell them to him for $50 each."
The villagers rounded up all their savings and bought all
the monkeys for 700 billion dollars.
They never saw the man or his assistant again, only lots and lots of monkeys!