Here's why:

* Incomplete dominance occurs when neither allele for a trait is completely dominant over the other. Instead, the heterozygous offspring exhibit a blended phenotype, a combination of the two parental traits.

In this case:

* The black mice likely carry two alleles for black fur (let's say "BB").

* The white mice likely carry two alleles for white fur ("WW").

* When crossed, the offspring inherit one "B" allele and one "W" allele ("BW").

* Since neither black nor white is completely dominant, the offspring express a mixture of both, resulting in gray fur.