1. Physical characteristics: This includes things like the organism's structure, form, size, and appearance. For example, a scientist might classify an animal based on its number of legs, wings, or the presence of fur.

2. Genetic relationships: This refers to how closely related different organisms are in terms of their DNA and evolutionary history. Scientists use techniques like DNA sequencing and comparing evolutionary trees to understand these relationships.

These two criteria, combined with other factors like behavior and lifestyle, help scientists create a system for classifying living things into different groups, called taxonomy.