Derek is a "flapper." When he gets excited about something, he jumps up and down and flaps his hands. Sometimes he runs back and forth or around in circles too. I like to think it's because he just can't contain his excitement. Maybe someday I will find out. Anyway, he usually does this when he sees something with wheels--such as a lawnmower, or a cement mixer, or a garbage truck.
Here is a video I took of him watching his father mow the lawn. Every time the mower went by, Derek was thrilled.