I like jokes, and I like it when there is something to them beyond a good laugh.
A soldier and a sailor are leaving a restroom when the soldier notices that the sailor did not wash his hands. He says:  In the Army, they teach one to wash hands after peeing! The sailor replies:  In the Navy, they teach one not to pee on one's hands! 
Indeed, fresh water is a scarce commodity on a ship.
4 mathematicians and 4 economists travel to a conference on Applying Mathematical Methods in Economics. The economists buy 4 train tickets, while the mathematicians buy just 1. On the train, when the tickets are checked, the mathematicians go together to a restroom, and when the ticket inspector knocks on the door, they open the door just little bit and stick out one hand with a ticket. The ticket inspector checks it and leaves. On the return trip, the economists buy 1 ticket, while the mathematicians buy no tickets at all. When the ticket inspector appears, the economists run to the restroom. The mathematicians knocks on the door. The economists open it just a little bit and stick out the ticket. The mathematicians grab the ticket and run to the other restroom. 
Indeed, mathematical methods are not always directly applicable to economics.
A group of scholars observe two bears entering an empty cave and three bears emerging from the same cave soon afterwards. Their explanations are:

Indeed, only a mathematician will explain this observation in terms of negative numbers of bears.
Sam Steingold<sds@gnu.org>  created: 20050805 