Categorical imperative
Thirsty Man has not yet opened the water bottle.  The new man has not had anything to drink for many days.  The second man is clearly dying of dehydration.  If he is not given water soon he will die.  We can call this second man, "Dying Man."  Thirsty Man now has a decision to make, will he share the water or drink it himself?  Thirsty Man does not live by the maxim of "I will drink water when I am thirsty," because that maxim fails the test of being universally fulfilling the categorical imperative.