how do penguins count as birds if they can not fly?

Question: how do penguins count as birds if they can not fly?
- Keywords:
  bird,
  penguin
Asked by Einstein to Alex on 13 Mar 2017.
- Alex Evans answered on 13 Mar 2017:
  
  Yep! There are lots of birds that can’t fly that are definitely still birds. All birds are thought to have evolved from flying ancestors, so even though penguins can’t fly now, their ancestors will have done!
  
  In fact, all living birds are technically dinosaurs! So the next time you see a robin or a chicken or any bird, you can say that you’ve seen a real living dinosaur!

Question: how do penguins count as birds if they can not fly?