A physicist is complaining to a mathematician about the high stress of his job. Experiments going wrong all the time, the high cost of replacing outdated equipment, hazardous working conditions, etc.
The mathematician suggests he take up fishing. "I fish all of the time. I find it to be very relaxing. In fact fishing is becoming the national sport of mathematicians world wide. Why don't you go fishing with me next Saturday?"
"Thanks, I will. But why is it so popular with mathematicians?"
"Well, when people see me sitting next to a pond, holding a fishing pole, they know I am fishing. So they don't every bother me because they don't want to scare away the fish. And just before I drop my line in the water I cut the hook off - that way the fish can't bother me either. That way I can think about difficult math problems in peace and quiet!"