Another oldie but goodie:
A koala bear was strolling down the street when approached by a prostitute. He had never been with one before and was curious, so off they went to a seedy, hourly motel. He went down on her one last time before departing, and as he was leaving the prostitute yelled, "Hey! What about my money?" The koala turned, gave her a puzzled look, and shrugged his shoulders. She pulled out a dictionary and pointed to the word "prostitute" and its definition: "has sex and gets paid". Finally understanding, the koala grabbed her dictionary, turned to the word "koala", and showed her: "eats bush and leaves".