Nasreddin once had an old boat, which he used to ferry people across the river. One day, he was taking a university professor to the other side.

"What is the square root of 9?" asked the professor.

"I don't know," answered Nasreddin.

"How do you spell elephant?" asked the professor.

"I have no idea," replied Nasreddin.

"Didn't you study anything at school?" demanded the professor, surprised.

"No," said Nasreddin.

"Then you wasted half your life," said the professor.

Nasreddin was silent for a little while. Then he said:

"Can you swim?"

"No," said the professor.

"Then you wasted ALL of your life," said Nasreddin. "We are sinking."