How to differentiate?

 Long ago there were a duo of father and son . Son could not find any work to do after so many attempts .

Father starts to worry about the future of his son after his death. Soon his health got deteriorate and he was counting his last days . 

That time, he called his son near to him and gave two glowing and shining stones . Both of them are looking the same . One of them is a diamond and the other one is made by glass .

It is very difficult to differentiate between the two . So father explained the fact and advised him to go near King's with the challenge to differentiate the diamond and glass out of the 2 stones . If they fail to do so  then the opponent must give the value of diamond to son.

After his father's death, son went near the king of his country  . He presented the 2 stones before him . He explained the challenge to King . King agreed to the challenge and called a famous jeweller who is a expert in finding out the quality of diamonds . 

The jeweller came and he inspected both of the stones . But he could not differentiate between diamond and glass . He accepted his defeat . Many jewellers came but no one knew the answer  . 

Son was awarded the money as promised and he moved to the next kingdom . There also same thing got repeated and son had won the challenge .

Now son has got enough money to spend his life luxuriously . But he continued the same challenge and went to different kingdoms . One day he came to a kingdom where he could say that it is a poor kingdom .

He went to the court . The king was so poor that he was running his court on an open ground . Son put the 2 stones before him and challenged the king and his courtiers to differentiate between them ,diamond and glass respectively.

Soon one old man got up . Everyone were looking hopeful at old man . Old man held both the stones under the sunlight . After some time, he separated them as diamond and glass correctly.

Everyone was surprised . Old man explained , " When kept under the sun , a stone which is made of glass will gets heated up . But a diamond will not ".