Languages (so I'm told).
What Phil said...
A foreign language
A second language, how to read music, but really just about anything. When we're little we don't reject new knowledge in the same way we do as adults.
Learning to walk
A new language.
A new language, for a fact. Adults are so set in their ways, and closed minded that it is much harder for them to grasp.
Video games, swimming, gymnastics, how to set up TV and sound system. :)