"What are that?"

All of a sudden, Parker has started acting really curious about things and asking what everything "is"! He repeatedly points to things he sees when we're out and about and says "What are that?" or "What is those?". We've been trying to teach him that if he is wondering about one of something we say "What IS that?" and if he is wondering about more than one of something, we say "What are THOSE?". Today he finally started to (sometimes) use them correctly! He saw a mark on his wall (from where his crib scratched the wall when we were taking it apart/moving it out of his room...oops!), and asked "What IS that?"! I love that he's gotten to this super curious stage of development! I DO however know that pretty soon he'll be asking "What are that?" to something that I don't know how to explain...then what?? I'm thinking I may end up doing a lot of googling in the near future to try to figure it all out...