Here's why:

* Genotype: The genetic makeup of an organism, represented by the specific alleles (versions of genes) it possesses. This is like the underlying code of the organism.

* Phenotype: The observable physical and biochemical characteristics of an organism, resulting from the interaction of its genotype with the environment. This is the expression of the code, what you actually see.

So, while the genotype is the blueprint, the phenotype is the built structure.