I was just reading last night about toddler's development in the latest Parents magazine, which said at 18 months old, kids tend to get frustrated easily because they want something but aren't always easily understood, which makes them pretty moody. Eli is a month away from the official time line, but I've noticed this week especially that he is either VERY happy when I understand what he wants or VERY upset when I don't.
When I do figure out what he's asking for, my favorite thing is that his face lights up, he claps his hands, and he says, "YEAH!!" with a big smile. It makes the frustration all worth it :)