A lecture about English
A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."

Advice from lawyers
George and Lenny decide to cross North America in a hot air balloon. However, neither were particularly experienced balloonists, and Lenny's mind quickly drifted from navigation to thoughts of how clouds look like cuddly little bunny rabbits. Upon realizing that they were lost, George declared, "Lenny -- we are going to have to lose some altitude so we can figure out where we are."

George lets some hot air out of the balloon, which slowly descended below the clouds, but he still couldn't tell where they were. Far below, they could see a man on the ground. George lowered the balloon, to ask the man their location.

When they were low enough, George called down to the man, "Hey, can you tell us where we are?" The man on the ground yelled back, "You're in a balloon, about 100 feet up in the air."

George Called down to the man, "You must be a lawyer." "Gee, George," Lenny replied, "How can you tell?" George answered, "Because the advice he gave us is 100% accurate, and is completely useless".

The man called back up to the balloon, "You must be a client." George yelled back, "Why do you say that?" "Well," the man replied, "you don't know where you are, or where you are going. You got into your predicament through a lack of planning, and could have avoided it by asking for help before you acted. You expect me to provide an instant remedy. The fact is you are in the exact same position you were in before we met, but now it is somehow my fault."