Eli5: why are some prehistoric reptiles not dinosaurs?

Question

I’ve gone my entire life being told “that’s not a dinosaur that’s a (insert what it is) but why? What’s makes one giant lizard a Dinosaur but a different giant reptile not a dinosaur?

In: 103

Answer 1

It’s the hip shape. Dinosaurs are the ancestors of modern birds and have hips like a bird. Prehistoric reptiles are ancestors of modern reptiles and have hips like a reptile.