One story that I heard was about an aquarium owner. 

I don’t know why but he put a barracuda in the same tank as a goldfish. 

Under normal circumstances, a barracuda would devour a goldfish in no time. But this tank didn’t have  the normal conditions. 

There was a clear glass partition separating the goldfish from the barracuda. Like a snake ready to strike, the Barracuda would circle round and round until he felt the  moment was right—and then BAM! He would smack the glass wall like you may have when you tried to  walk through a recently cleaned sliding-glass door. There was no meal for him this time. 

A couple of hours later, he would try again to no avail. And the next day, he would fail again. The  aquarium owner got used to the thud sound of the barracuda hitting the partition. 

Then there was a day where the owner heard no sound coming from the tank. He checked and saw that  both the goldfish and barracuda were doing fine—it was as if the barracuda had given up. 

So the owner removed the partition. 

And the barracuda could have had success, if he just tried one more time.  

But he never did.