This morning, on the typically 35-45 minute drive to the girls' school, my 5 yr old said that she learned in preschool (in previous years) that "extinct" means to go away and never come back. I confirmed to my 5 year old and my 7 year old that for something to be 'extinct' is for all of its kind to go out of existence and to not come back. And that seemed to satisfy them as well as their curiosity.
Would be rather inconvenient and dangerous for us if certain creatures, say dinosaurs, were able to come back out of extinction.
The questions and comments my four children come up with bring smiles to me and make my day!