Basic Submarining.

If ever you find yourself invited onto a submarine here is the one thing you must do.
Carefully count the number of times you dive.
Carefully count the number of times you surface.
Add them all up and divide by two.
Now, if after that you have any left over DON'T OPEN THE HATCH!
Carefully count the number of times you dive.
Carefully count the number of times you surface.
Add them all up and divide by two.
Now, if after that you have any left over DON'T OPEN THE HATCH!