Two friends were walking through the desert. At some point of the journey they had an argument, and one friend slapped the other in the face.
The one who got slapped, was hurt, said nothing, and wrote in the sand:

"Today my best friend slapped me in the face."

They kept on walking until they found an oasis and decided to take a bath. The one who got slapped in the face got into trouble and almost drowned, but his friend saved him.

After he recovered, he took a chisel and with it wrote on stone:

"Today my best friend saved my life".

The friend who had first slapped him and then saved his life, asked:

"When I hurt you, you wrote in the sand and now you write on stone. Why?"

The other friend replied:

"When someone hurts us, we should write about it in the sand so that winds of forgiveness can blow it away. But when someone does something good for us, we should engrave it into stone so that nothing can erase it.