A puzzling issue for educators in elementary education is how to explain arithmetic operations between negative numbers: why, for example, subtracting a negative number is the same as adding a positive number or why the product of two negatives gives a positive. A related problem is to find good examples in which these operations appear in the "real world" and that students find sensible.

Here the immersion of a submarine to a certain depth is interpreted as a negative quantity, where the surface of the sea is taken as zero depth; a change from an initial depth to a final depth requires manipulation of negative quantities. When the submarine dives, the result of the subtraction corresponding to the difference of depths is negative and when the submarine rises this result is positive.