How to tolerate pain?

An aging master grew tired of his apprentice’s complaints. One morning, he sent him to get some salt. When the apprentice returned, the master told him to mix a handful of salt in a glass of water and then drink it.
“How does it taste?” the master asked.
“Bitter,” said the apprentice.
The master chuckled and then asked the young man to take the same handful of salt and put it in the lake. The two walked in silence to the nearby lake and once the apprentice swirled his handful of salt in the water, the old man said, “Now drink from the lake.”
As the water dripped down the young man’s chin, the master asked, “How does it taste?”
“Fresh,” remarked the apprentice.
“Do you taste the salt?” asked the master.
“No,” said the young man. At this the master sat beside this serious young man, and explained softly,
“The pain of life is pure salt; no more, no less. The amount of pain in life remains exactly the same. However, the amount of bitterness we taste depends on the container we put the pain in. So when you are in pain, the only thing you can do is to enlarge your sense of things. Stop being a glass. Become a lake.”