Are platypuses mammals if so why do they lay eggs and have beaks?

Question: Are platypuses mammals if so why do they lay eggs and have beaks?
- Keywords:
  animal,
  mammal,
  platypus
Asked by 866smap42 to Sheila, Piyush, Natalia, Gary, Dimitar on 7 Nov 2018.
- Gary Munnelly answered on 7 Nov 2018:
  
  I don’t know the answer to this, but it did remind me of this picture. So here you go:
  
  You’d need to ask a biologist of some description about this, but it’s definitely an interesting question.

Question: Are platypuses mammals if so why do they lay eggs and have beaks?