Story Puzzle 2: Door Puzzle

You first meet Luke in his room. However, he has locked himself away, and won't open the door for you. He tells you that surely the great Professor Layton will have no trouble figuring out this puzzle. There are a number of items around the door you can click on and learn more about. However, something around the door is odd. When you are ready, you tell Luke, who gives you 15 seconds to figure out (and tap) the item that is odd. Once you are successful, Luke will open the door and talk to you.
This is both a visual puzzle (which of the items around the door are important?) and logic (what makes the item important? What is Luke really asking you to do?) The 15-second time limit is an interesting twist to keep you on your toes.