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!

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.

A group of scholars observe two bears entering an empty cave and three bears emerging from the same cave soon afterwards.

Their explanations are:

Physicist:

Experimental error.

Biologist:

One of the bears was pregnant and gave birth in the cave.

Mathematician:

If now one bear enters the cave, it will be empty again.