Chinese Old Story : sand and rock

this is an old chinese story :

someday there were 2 boys playing in the garden.. one of them fell and got hurt.. so the other helped him.. the falling boy then wrote on the rock ” Today my friend helping me”
the other boy didn’t know why the boy wrote those but he didn’t care so much about it.
Then another day, that boy who has helped the other make a mistake to the other.. he lost the other boy book… he felt sorry.. so the other wrote again, now on a sand, ” Today my friend has lost my book”…
Then his friend asked why he wrote on rock when he helped and why he wrote on sand when he made a mistake..
the writing boy answered : i wrote your kindness on the rock so that it will never go away and i always remember, but i wrote your mistake on the sand so that it will go away with the wind..