The CIA once sent a mole, posing as a Russian student, to one of Moscow’s technical universities. Two months later he was uncovered.
"Well," the spy explained to his superiors at the American embassy, "I couldn’t drink that much. Firstly, it was almost physically impossible, and secondly, even if I could, it would have been against your instructions."
The CIA changed the instructions and sent in another mole with the same orders. But three months later, they were facing a similar disaster.
"Have you been drinking like everybody else?" they asked the mole.
"Yes I have," the mole replied.
"How did they get onto you then?"
"They got suspicious when they found out my attendance rate was the highest in the entire university," the mole explained, "but I was only following your instructions."
The CIA changed the instructions one more time, and they sent in their third mole. Unfortunately, during the mid-term exams, they received the devastating news of his failure. When the expelled "student" arrived at Langley, he explained, "I had been drinking and partying just like everybody else, and just occasionally showing up at lectures."
"How did they get you then?" his superiors asked.
"I was the only one who failed the exams."

A student is floundering during an exam. "Your mind is like a desert, sir," the professor tells him in frustration.
"Every desert has an oasis, professor," the student replies. "But not every camel is able to find it."