"Pencil Between Thumbs Trick"
This is a little trick I sometimes do as a "warm up" for the students.  I challenge them to place a pencil between their thumbs and then "move it underneath." 
It is a bit of a challenge at first for many, but once I walk them through it by doing each part of the movement, it becomes much easier. Then after they've done it once they tend to remember it.  This activity develops dexterity and visual tracking skills.