Forbidden Fruit

A poor farmer had a lot of mouldy hay.  Instead of wasting it, he tried to feed it to his cows but the cows would rather go hungry than eat the bad tasting grass.

So the farmer mixed the hay with some fresh hay and gave it to his cows.  The cows simply separated the good hay from the bad and ate the good stuff.  Still the mouldy hay remained.

Then the farmer noticed something strange.  Even though there was plenty of grass in the paddock, the cows would often be seen pushing their heads between the wires of the fence to eat the grass just outside the paddock.  So the farmer left the mouldy hay just outside of the fence, but close enough for a cow to reach with a stretch.  The mouldy hay was all eaten in a couple of days.

Forbidden hay, even when it is mouldy tastes sweet.

Or the grass is always greener on the other side of the fence.