Answer #3

One of these things is not like the others

new
old
full
half

If you haven't had a chance to work on this one yet, you should go to Question #3, to avoid the text below (it contains the answer).

What I like about these "not like the others" puzzles is that sometimes they become more difficult the more you know. Some, like this one, are perfectly obvious after you find out the solution, but seem particularly difficult to solve beforehand.

Three of the items on the list are phases of the moon. (At least, as they are colloquially called.) "Old" is not a moon phase so it does not belong. It is not like the others.