What are two features that a mammal has in common?

Here are two features that all mammals have in common:

1. Mammary glands: This is the defining characteristic of mammals. Females produce milk to nourish their young.

2. Hair: All mammals have hair at some point in their life cycle, even if it's just a very fine coat. Hair provides insulation, helps with sensory perception, and can be used for camouflage or display.