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

I may be way over simplifying it but the way I explain to high schoolers how all dinos are reptiles but all reptiles aren’t dinos is: dinos are terrestrial, have legs under their bodies not sprawled out from the sides, & laid hard shelled eggs in nests. These 3 characteristics exclude all the groups I used to think were dinosaurs when I was a kid!