Ethan's First Sentence

I'm not really sure what Ethan's first word would be considered.  Ma (Mom) and Da (Dad) have been around for a while, but he hasn't said anything else that I recognized yet.  Today he climbed on a chair by the kitchen table and when he was there and discovered that his water bottle was on the other end of the table he hollered, "Ma!" and when I looked at him he pointed and hollered again, "Da (that)".  I'd declare that his first sentence.  Is it legal for "that" to be a first word after mom and dad?  It isn't very exciting, though it is in a way...