Answer:
There are two things which will tell you if an animal is a mammal or not.
1. All mammals make milk to feed their their young. Only mammals do this.
Don't be fooled by breasts and nipples. Some mammals have neither of these - just a special patch of skin which gives milk when baby licks it.
2. All mammals have three tiny bones in the ear, called the hammer, anvil and stirrup. These bones are what give mammals their very good hearing. Only mammals have them.
Many people think that all mammals give birth to live young. Not true. There are three species that lay eggs!
It is true that mammals are hairy, but so are some insects.