Heavy Rock

Some of Nasreddin's old friends were talking about the young people in their town.They all agreed that old people were wiser than young people.Then one of the old men said ,But young men are stronger than old men.
All of them agreed that .this was true ,except Nasreddin,He said ,No.I am strong as when I was a young man.
What do you mean ?said his friend .How is that possible ?Explain yourself !.
Well,said Nasreddin,in one corner of my field there is a heavy rock.When I was a young man I used to try to move it,but I couldn't because I was not strong enough.I am an old man now,and when I try to move it,I still can not !.